Ubuntu Login экран выводится на другом мониторе

This is a known and reported bug with gdm3. The current workaround appears to be:

  1. Go into Settings > Devices > Displays and configure your monitors the way you want for your login screen (in your case, internal laptop display disabled). Click the «Save» button when done.
  2. Copy your user’s monitors.xml file into the home folder for gdm user.

To copy the monitors.xml file, open a terminal and perform the following: