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

Step 4.
if that doesn't solve the problem or result in other problem, remove the buggy package

sudo remove xserver-xorg-video-intel

and reboot

ref

• SO - Intel screen tearing - Ubuntu 18.04
• Fix Screen Tearing in Linux