I finally found out what is causing it. It is oUF Aura Tooltips.
I created a nameplate addon using oUF and I configured the auras to have enabled mouse events:
Lua Code:
auras.disableMouse = false
So what's happening now is the following:
1. Tooltips are not leaving screen:
2. I hover an aura on a nameplate to check the aura tooltip
3. Tooltips are now leaving screen:
After a UI Reload this is fixed again.
Does somebody else have this problem with oUF Auras and knows how to fix it?