The album art viewer (the window that opens when double tapping the thumbnail) seems to open in the position it was last closed, which unfortunately may cause the window to be invisible if it was last closed on a secondary display and then this monitor is removed.
Quite a minor bug, but I am usually affected by it more often than normal because I travel with my laptop and plug it to a external monitor when at home. I am using MusicBee v3.1.6512, Windows 10 x64.