Issue with wrong normals only on the portrait frame

[Jaccouille]

Hello,
I got an issue with some of my custom models, the normals appear to be wrong only on the portrait frame (see linked screenshot, portrait is darker than it should be).
If I select another model, who do not have this issue and then select my problematic model, the issue is gone (portrait no longer darker).

I am used to do "recalculate normals" and "flip all selected normals" in Retera model studio although I do not understand the "science" behind it, as long as it worked I was happy.
I once managed to fix it but it was completely random and I don't remember what I did.

It is a minor problem, but it somewhat bothers me as I don't understand why it's happening, if someone has an explanation for this behavior (I might as well just summon @Retera since he never fails to answer on those subjects)

Problematic model can be found in the attached files.

 

[Jaccouille]

Update: I managed to fix the issue by deleting the current camera object and creating a new one from magos.
The faulty camera object was imported from another model file using Retera model studio, maybe there's a slight issue with the import process?

I compared both camera object in the MDL files but nothing seemed out of place for me a first sight, I'll look more into it later.