If I swap out GPUs, the system works fine. So it seems to be the GPU at fault. This GPU has a switch for overclocking (two sets of BIOS). I switched it while the system was powered off multiple times testing things and this is the situation I’m in now.
I’m not sure if the symptom (only booting to a black screen with a _ and never going on to the splash screen) sounds like it’s caused by what I think it’s caused by.
I’ve had similar symptom with a Sapphire Nitro RX 480 with a dual BIOS. Silent BIOS booted. Turbo BIOS wouldn’t. Turned out the motherboard UEFI implementation was old and not fully compatible so I disabled the UEFI booting I think or at least played with the legacy options and it would boot to Windows but no splash screen or BIOS prompt.
Just a possibility a BIOS option has changed. Otherwise it still may be possible to flash the GPU BIOS if you can get it to boot in another system as a 2nd card? I could do it easily with RX 470/480s but I’ve no experience with other AMD GPUs.
The system has a RyZen 1700 CPU which has a 65W TDP. The Radeon Pro Duo has a 375W TDP. I’m not overclocking either. They’re both water cooled, so they’re both more efficient than with air cooling because they stay sub-30C on idle and sub-60C on load (from what I’ve tested).
I should be using about 75% of the total available power output of this PSU, so I wouldn’t think this is the problem.