(On Windows 10.) I know that Lightroom is notorious for not letting you decide which of your secondary monitors it should use. I have 3 monitors connected to my desktop, they stand in this order   
 (Display port) Cheap "4K-supposed-to-be-but-not-truly-colour-graded" monitor. I use it for the main Lightroom Classic window.
 (Display port) "Better graded 4K monitor". I use to display the preview.
 (HDMI) Random 10 years old Full HD television which is mostly there to play YouTube videos or other secondary stuff while editing photography.
I haven't done any change (to my knowledge) to anything in the system, but since today, Lightroom is displaying its preview window on monitor 3.
Since Lightroom has been able to do that change the monitor it uses... any idea how to change it back?
(Changing the monitor placement is not an option considering the shapes of my desk, of apartment and of Human neck. Virtually changing monitor position is an option I though out but I don't want to get wrong "display connexions" so that would be a last resort solution.)
(Also, I noticed that the preview Windows now has a "Full screen"/"Windowed" mode... No idea if that would be the way to a satisfying solution, but I switch it to "Windowed" mode, it is display on the Full HD television with 4K dimensions, so I can't grab that window and move it to another monitor so far.)