This could be an issue with OpenGL 2.1 support in GZDoom...
Now, why would that matter for a GPU that supports GL4.5? Simply because Mesa is
absolutely autistic and sets the default OpenGL profile to 3.0 instead of 4.0 or the highest the card supports. Since GZDoom's minimum requirement for the modern GL3+ renderer is OpenGL 3.3, it gets kicked to the legacy OpenGL 2.1 renderer.
This WAS supposed to be fixed/handled in GZDoom's side, but maybe the fix doesn't work for all GPUs or all versions of Mesa. Try forcing Mesa to use the OpenGL4.5 profile and see if that works.
Maybe this will helpShouldn't be one of those occasional AMD driver bugs, since you're using Mesa (hence why I think it might be what I said above). But maybe it could still be that
somehow, who knows.
Or maybe Mesa's support for your GPU is broken, somehow.
So yeah, this is most likely a driver bug of some sort.