Comment # 8 on bug 109001 from Michel Dänzer

I suspect disabling V-Sync in xfwm4 just avoids the problem, i.e. it's a
workaround, not a fix. Unless you're using rotation or other RandR transforms,
leaving V-Sync enabled and leaving TearFree at the default (auto) might also
avoid the problem, and would be more efficient.

Does
https://gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu/merge_requests/15
help by any chance?