PS: Please also send the findings to ASRock’s Tech Support. It’s not about rubbing it in but hopefully they’ll take notes from this exercise to maybe in the future be able to respond to a similar request from another user more usefully.
I might be able to also check ECC on an ASRock X570 Taichi, was my first X570 motherboard before moving to the ASUS ProArt X570-CREATOR WIFI because of better IO and more frequent UEFI updates.
Thank you very much! Now not all hope is lost. 
This means that the 5700X should not be a problem. Now all that’s left is for me to try to get this working on the Taichi as well.
I will also try my best to check on my Taichi, however that will probably take some time, as I do not have much experience in overclocking/undervolting RAM. Anyway, since looking at your screenshot it seems that it may take half a day or more for a single error to happen, I will try to get back to 4000 MHz and 1.1 V and then leave MemTest PRO running for a while to see if everything is OK. Just out of curiosity, what was the last working voltage in your case?
Yes, I will definitely get back to ASRock’s Tech Support, especially since they did not even bother replying to my last e-mail. Hopefully, they will be able to use this information to improve their service in the future.
Well, I’ll be d*mned , but it seems that you are even using similar if not the same memory sticks. Even though I cannot see the voltage on the screenshots, I thought that maybe I could get an idea if I had a look at your timings to see how far they were from the ones I tried last time, but they were too similar. I am using a Crucial 32GB RAM as well (TrueNAS shell says 18asf4g72az-3g2f1 under part number, should be the MTA18ASF4G72AZ-3G2R model though).
The memory details shown on MemTest86 are just the default part names and their default SPD profiles, not the actually active settings.
Voltage settings cannot be really be applied 1:1 between different motherboard models, manual trial-and-error is required.
The “winning” configuration with the 5950X was memory at DDR4-3600, Infinity Fabric at 1800 MHz, CPU voltage via offset - 0.01875 V and memory voltage at 1.18 V.
The other way around was not fruitful, DDR4-3600 seemed to have been the limit with my parts, even after going to 1.35 V above DDR4-3600 wouldn’t POST and settings that would POST didn’t produce any memory errors within a few hours.
Memory tests that are somewhat robust and can withstand criticism take many, many hours, in my previous life before ECC errors in MemTest86 would only appear in Pass 3 or 4 after a day or so.
That is true.
Thank you for the values as well. I will try looking around by starting from those values and see what I get. So far I had no luck with 3800 MHz and 1.178V but as you said, we will have to wait and see.
I had memtest report to me that ECC corrected a single bit error on one test once with my 5800X3D on an ASUS Pro WS X570. So I’ve seen it work once, but usually it’s just “no news is good news.” 
Thank you @GeorgePatches! Honestly, judging from these facts, I would bet that the 5700X should then also report ECC errors (as per AMD’s statement).
The only thing left now is to find out if the Taichi reports the ECC errors.
As an update, I have been running a (sadly a bit short) memtest with the following options: 1.14V and 3800 MHz. The system would not boot under 1.14V so I thought this might be enough. Sadly, I did not get a single ECC error in Memtest PRO 10.3 in 4.5 hours. I will try to further reduce the voltage and increase the frequency and run a longer (cca. 12 hours) test and report my results.
Instead of playing with voltages on the RAM, just use a heat gun on low aimed at the ram from about a foot away.
Pretty sure the RAM will puke errors after 5 minutes.
That’s a nice method however I didn’t want to risk anything happening to my systems. I really appreciate an issue counter of 0 in day-to-day usage.
Then why an exception for undervolting/overclocking?
I can almost set all BIOS settings blindly and with BIOS flashback there is practically 0 chance of messing components up permanently.
Thank you @KeithMyers for the idea!
Honestly, I have read somewhere that heat guns (similar to hair dryers) can generate static electricity, which may damage the component and/or the motherboard, so I did not opt for that option. However, following your advice, I found that basically a simple lamp directed at the RAM can do the same (i.e. generate heat & light) while not being dangerous. Therefore, I used a lamp and directed it at the RAM sticks. I could raise the temperatures by 15 degrees, hitting 60 Celsius at the end. However, still no ECC errors so far.
The setup that I am now using (and used for the lamp trick as well) is:
1.098 V (the minimum that the BIOS allowed) and 3733 MHz. With this option, the computer would not POST in 50 to 66% of the cases and even if it did, the loading usually took some time. Therefore, I chose this for running the tests.
So far, I am running my 3rd pass with 6 hours into the testing. Sadly no ECC errors so far (not even with the lamp).
I will run this test to completion (4 passes) and post the results.
I’ll try tightening tRCDRD, maybe to 16 ticks or even 15 ticks. Basically only samsung B die can do very low tRCDRD and anything else should fall flat on their faces.
Okay, so the results are out: no ECC errors reported so far.
I have run Memtest for a total of 24+ hours combined with various settings that would barely boot (4000, 3800, 3733 and 3600 MHz with various voltages). Still no positive results, even after turning to the “lamp trick” and heating the RAM sticks to 60 °C.
Today, I benchmarked with the previously mentioned 3733 MHz and 1.098V setting, which would often not even post (in fact the PC was not even able to restart after exiting from Memtest and selecting the Reboot option). Yet, zero errors.
Now I am really starting to question if ECC reporting works at all on this motherboard.
EDIT: Since the system would revert to the default settings (3200 MHz and 1.2V) upon failing to boot, I deliberately checked the BIOS settings before running Memtest to guarantee that Memtest is being run with the custom settings and not the default ones.
From the various RAM overclocking threads at OCN, the trip point for DDR4 and DDR5 RAM temps is around 65° C. before they starting generating errors in Memtest and the other RAM testing softwares.
I don’t think your lamp is getting them to the temps that generate the memtest errors. Why I am sure that the heat gun would do the trick. I’ve never heard of any example where a heat gun generated static electricity. Leaf blowers on the other hand . . . . . yes.
Do you have 2 or 4 DIMMs installed?
Well, I’ll be d.mned, but I cannot see any articles, when they deem heatguns as dangerous either, only hair dryers. I thought that since they operate in a similar way, the same issues would be present in the case of the heatguns as well. However, it seems that I was wrong, as even ChatGPT says that it cannot generate static electricity. If push comes to shove, I will buy a cheaper, low-powered heatgun and see what happens.
@aBav.Normie-Pleb
I am using 2 RAM sticks in dual rank, so only 2 DIMMS are populated on the MB.
I’m about to succumb to curiosity and might test my old X570 Taichi with older “not important” DDR4-2666 ECC memory with a hot-air station (Weller WTHA 1, intended for soldering) soon.
That really, really implies that ASRock has messed up previously working ECC reporting on AM4, too - similar to their mess on AM5 
(But it’s still in speculation territory)
But I’m already getting Broadcom vibes from your ASRock Tech Support experiences, which is one of the harshest things I am capable of uttering.
@aBav.Normie-Pleb Thank you for your helpful intentions!
Honestly, this is why I did not edit/change my previous stance: I do not recommend this board to anyone, and that will not change even if I manage to get it to report the ECC reports.
The lack of knowledge and skills of ASRock Support, as well as their lack of communication, willingness to help, the super long reply times, and trying to point fingers at anything other than the MB (e.g. AMD, or the RAM itself) instead of actually taking the time and looking into the problem, as well as keeping the consumer in the dark for prolonged periods of time by promising things that they will not do (in my case they promised to directly contact AMD and ask about the 5700X about 2.5 months ago, which they did not do to this day, they also promised to have a look at the x470/x570 ASRock Racks about 2 months ago to see if they report the errors as opposed to the Taichi, which they did not do, etc.) ) simply make me even more sure not to buy any ASRock products in the future, even though this was my first product from them. I am especially salty about the last item (promising things and then simply forgetting about the consumer), as I deliberately contacted them as soon as I built the NAS and tested the functionality, so that I could send the MB back if it cannot do what it is being marketed of being able to do and then get another MB (for example ASUS), yet here I am, 3 months later, with the same MB and the same (if not more) questions.
Alright, so I went and bought a brand new heatgun to test if Memtest shows any ECC errors.
Just to be sure, I did the same overlocking/undervolting and used the following settings:
3733 MHz and 1.098V
Using these settings, I used the heatgun to directly heat up the RAM modules. I heated them up to 73 °C and the CPU reached the same temperatures as well. Then I stopped heating it up further and instead moved the heatgun further to kind of stabilize at 70+ °C. Then I ran Memtest for 30 minutes.
Results:
Still no ECC errors
Are you seeing errors that memtest finds?