It’s a known I received many feedbacks on this particular issue, for example this one
It only happens on the Air 4 (that I don’t have).
Can you confirm the bug is still happening when you have:
- smoothing shading on (Settings menu)
- no lights/shadows
- try disabling “Buffer reuse” in the Interface menu
Otherwise rebooting completely the iPad usually fix the issue from what I heard, but only temporarily.