Are you certain the Vega56 is still healthy and stable?

It's possible it's just KDE, but it's also possible KDE is triggering hardware issues.

RAM is also always one to check, especially on an older machine.

Might be a driver issue, too. I remember Vega as a sort of short-lived in-between generation with especially many driver problems.

Actually, early Ryzen 1800X also crashed under certain workloads on Linux, especially compilation and downloading games on Steam(!) IME - another KDE guy and me were some of the first world-wide to communicate about the problem. AMD had a hardware replacement program, maybe it's still active.

https://www.phoronix.com/news/Ryzen-Segv-Response should get you started. People made special software packages to trigger the hardware problem in a minute or so.

There was IIRC another problem with early Ryzen, something about transitions from idle power causing instability, fixed by essentially raising idle power consumption a little with a special BIOS switch and / or playing with load line calibration. That one crashed the whole computer, not just the offending program. (Actually, crashes while downloading games on Steam might have been that one.)

And yes, do run memtest (from the boot menu) for a couple of hours or over night, too.