aha. I'd thought that's what you'd have had from the beginning.
Then that actually explains more. The kernel module and xorg module should be from the matching series.
If I'm understanding this correctly, then you're also using the "radeon" kernel module, and then that's how it's supposed to work as I'm reading it...
xf86-video-ati is the xorg module for the "radeon" driver (as I said, I'd previously misspoke when calling it "radeon"). xf86-video-amdgpu is for the new AMDGPU driver.
While it should have worked between xf86-video-amdgpu and AMDGPU, you might still be suffering a bug on that one since you did have it matched and still had problems, but at least you've got a working setup now.
Sounds like a flaw in Xorg if it can attempt to use the wrong xorg module for the loaded kernel module. But I don't know enough about how that works...
Needless to say, if you're somehow using the "amdgpu" kernel driver with the "ati" xorg module, then I'm clearly thoroughly confused as to how this works!