Summary: My external monitor is disabled after power save turns screens off, changing the multiple screens setup, and remains disabled until I press a button on the monitor.
I have a monitor connected to my laptop via DisplayPort. Whenever the power save settings lock the screen, both the monitor and laptop screen enter power save mode, as would be expected.
However, right after this happens, the monitor is disabled, and the screen setup changes to use only the laptop screen ( the laptop screen switches on, and its resolution changes before being turned off again after a few seconds, as it is in power save mode).
Then, when I move my mouse or press a key, only the laptop screen is switched on and enabled, while the monitor remains in power save mode. It is only enabled again if I press a button on the monitor itself.
Ubuntu knows perfectly that it is there, because it shows in gnome "screens" settings, and I can enable it back there again ( although I lose the resolutions and position preferences if I do that).
This would be just a minor inconvenience if it wasn't because whenever this happens, all window positions in the monitor are rearranged.
If I use VGA with the same monitor, it doesn't happen (but then I can't use its native 2560x1440 resolution).
This also occurs when booting from a LiveUSB of Ubuntu 18.04 LTS.