How long is the cable between the usb-c to displayport cable and the KVM? How long is the cable between the KVM and the display?
That resolution + refresh rate is pretty close to the max bandwidth supported by the KVM, and by displayport 1.2, and can also suffer if you have lower quality displayport cables.
I have a similar display ([email protected]), and connect it to both a Linux laptop and macbook via a thunderbolt 3 hub without issue, but I was pretty selective with my cables. From wendell:
A quick test to run would be to try it with shorter cables. I also have a USB-C to displayport standalone adapter, and found that it can struggle to work with same-length cables that would otherwise work, so you might also have to take that into account.