Fullscreen not working anymore on 3rd display

Hi,

I have a Mac Pro with two thunderbolt displays attached to it, and and a benq projector to use Kodi on. It always worked perfectly. But today I replaced one of the TB displays with a dell 4K monitor. Now, kodi still starts in Fullscreen on the projector, but as soon as I start playing a video, it starts on the dell in full screen mode. If i then hit escape, the menu overlay pops up and everything switches back to the projector. Now the video will even play there, but with the menu overlay on all the time. I don’t know how to remove it after that.

In the settings, it states the playback display is the BenQ, and in 1080p.

So I’m really confused why this happens now. Maybe it has something to do with the refresh rate or something?
Any thoughts how to solve this?