Thanks for the tip. I've properly reported the issue.
I can say for sure it's not due to temperature. The GPU is a lot colder playing these games compared to many others I play.
It seems minimizing and maximizing the game on Path of Exile corrects the bug, but the same doesn't apply to Crusaders Kings II unfortunately.
Here's some other screenshots of the bug:
The portal light effect is completely messed up.
Minimap rendered unusable and the holvering shields over land are messed up.