A version of linux-firmware should be installed. Please try this:
sudo pacman -Q --info linux-firmware
The package contains GPU specific firmware files which are used by AMDGPU.
Intersesting finding from the linux-firmware repo: on Aug. 11th there was a commit with the following commit message:
amdgpu: revert back to older picasso sdma firmware
Newer firmware seems to cause random stability issues that
are hard to reproduce reliably to root cause. Revert
back to the older SDMA firmware until the issue is fixed.
Maybe this is the problem we are seeing with the newer firmwares.
Edit: Raven and Raven2 firmwares are reverted to older versions as well.