Lightroom Classic not displaying properly

Hi, I'm using Adobe on an iMac along with a XP-Pen graphics tablet. Up until recently I've had no issues with any of the Adobe programs. I use Lightroom Classic and Photoshop predominantly. Usually when I open LR it loads up on my XP-Pen tablet. Recently LR has now been opening up on my iMac screen. It loads full size with the traffic light buttons not showing. I cannot resize the window to my usual config. All I can do is go into Window and select centre. The traffic lights then show and I can drag it to the XP. But, I have to drag to about 50% of the way down on the tablet screen, go back into window and select centre or fill screen. But, when I then try to use a plug-in, or in fact open any other program on my iMac, LR immediately reverts back to the oversized version on the iMac. I can't figure out how to get it back to how it was before. Interestingly, Photoshop is not affected.. Any help would be appreciated. Photos are when opening and after dragging so LR is not showing on both screens at the same time. 
Thanks