Is this caused by a mod or just a bug? And how can I fix this?
I've noticed it most on the doors at Volkihar Keep and at lumber Mills.
What mods are you using. Sounds to me to be a mod issue?
I believe it might be the unofficial patch or the Skyrim Graphic Overhaul depending on what it does.
The thing is did you go back to a game save before installed the mod. It is best to keep that mod. Let Arthmoor know about the issue and wait for them to fix it.