Ubuntu 20.04 Screen tearing

Screen tearing may happen when you change a display orientation to "Portrait Right" or "Portrait Left"

If you are using Intel's built-in graphic, you can try the following method.

Step 1. If you don't have the folder "/etc/X11/xorg.conf.d", create one.
Step 2. create the file "/etc/X11/xorg.conf.d/20-intel.conf"
content in 20-intel.conf

Section "Device"
    Identifier "Intel Graphics"
    Driver "intel"
    Option "TearFree" "true"
    Option "AccelMod"    "uxa"
    Option "DRI"         "3"
EndSection

Step 3. reboot

ref

SO - Intel screen tearing - Ubuntu 18.04