The last time I had a weird issue like that it was a stale .Xauthority file in my home dir but I don't think I've ever seen that happen on upgrade.
I only use KDE on FreeBSD and raspbian with single monitors so no real insight there.
I haven't seen anything like that on Mint with Cinnamon, though.
Is it Xorg or Wayland? What login manager are you using?
