Yes, it's still happening. It's just running vkmark in default mode. It looks
like this: https://youtu.be/MngLl6BgOfg

I'm not sure if previously it happened continuously but now it's only a couple
of flips that fail (there's noticeable stutter visible when it happens).

Not sure how to make sure it doesn't use EGL but I ran it with
KWIN_OPENGL_INTERFACE=glx kwin_x11 --replace, I hope that's enough.

Currently on kwin 5.12.5, xserver 1.19.99.905 g4191b59bd and latest
xf86-video-amdgpu git so it has not been resolved there yet.

It happens with kwin's OpenGL 2 and OpenGL 3 backends and all vsync methods,
including None.
It does not happen with XRender.