I did some further testing and with Debian Stable (Stretch) everything is working fine and also the monitors.xml gets interpreted properly.
Also with Testing (Buster) the monitors.xml seems to be interpreted correctly (laptop screen as only connected monitor), tested with rotated login screen to the right. So it seems that gdm3 has problems to recognize the multi monitor setup or if lid is closed and one external monitor is connected, the external one. I also disabled in logind.conf all lid close actions so that the laptop did not suspend, but even then the login screen does not appear on the external monitors.
