So there is this problem that has been annoying me for ages.
My screen setup is made so that I have 2 portrait mode screens with 1 landscape screen in the middle.
If Spotify starts up fullscreen on the portrait mode screen, it will not go fullscreen properly on the landscape screen, it just cuts off the side (to the width of the portrait screen). Vice versa for the landscape screen.
It is just the actual display/rendering that doesn't show, the UI still completely works, for e.g. I can click on some space where there is no UI and still select a playlist or skip a song, etc.
However if spotify it not started up in fullscreen and is only windowed then it will go fullscreen on any type of monitor.