This has been an issue on other linux too. I am using a dell xps 13, so I am connected to the monitor through thunderbolt 3 to hdmi to my second monitor. The monitor almost always works initially IF I log out and back in (on logout screen, monitor turns back on), but after logging in, the display might still work for a short time, but often turns itself off (no signal – not showing as detected). Furthermore, if I plug my monitor out and back in, that does not change anything – still undetected.
Not sure how to troubleshoot or hopefully have the detection of the monitor not stop.