There’s a known (or suspected) bug in the AMD drivers, discussed here, with symptoms very similar to what you’re describing:
What they’re asking people to do is try this kernel:
There is actually an arch package that claims to be that amd-staging-drm-next version of the kernel, but to me it looks out of date; I think you may need to compile your own kernel if you’re comfortable if you want to try out that. I’m currently trying it myself because I’m having a similar issue; I can let you know what I find if you want.