Primary display randomly changes
This worked for me, credit to this answer:

https://superuser.com/a/1220526/454133


  Play around with moving the "top" bar between screens in the Arrangement window. Assign it to different screens and you will notice it will solve your issue once you find the "right" screen.


So, monkeying with the menu bar in the arrangement settings will reset your displays proper. I even got screenshots.