The bug is also effective on ubuntu 20.04.3 - fresh installation with all updates. But it wasn't effective prior updating.
After downgrading xserver-xorg-legacy and xserver-xorg-core from 2:1.20.13-1ubuntu1~20.04.3 back to the version 2:1.20.13-1ubuntu1~20.04.2, the problem seems to be gone. I'll come back in a week and confirm whether this is really true.
The bug is also effective on ubuntu 20.04.3 - fresh installation with all updates. But it wasn't effective prior updating. 13-1ubuntu1~ 20.04.3 back to the version 2:1.20. 13-1ubuntu1~ 20.04.2, the problem seems to be gone. I'll come back in a week and confirm whether this is really true.
After downgrading xserver-xorg-legacy and xserver-xorg-core from 2:1.20.