21.04: The screen is flickering

1. I installed http://linux.darkpenguin.net/distros/ubuntu-unity/21.04/testing-week/ubuntu-unity-21.04.iso

Effect: Now and again it happens that portions of the screen flicker quickly. The effect disappears after a couple of seconds when I continue to move the mouse cursor, but after a while the effect reappears. I haven't been able to record it on video because the effect disappears too quickly.

Only happens on a physical machine.

Also reported in #72 (closed) . The reporter had this issue after upgrading from 20.10. I have it on fresh install.