Thanks a lot for the answer. Anyway, I think this is not the problem.
These are the stats for this mesh zone:

As you can see, theer are 3 faces, 8 verts, 10 edges. Everything seems normal. I have tried to remove duplicates anyway but same problem appears.
To let you check it here is the blend file:
https://drive.google.com/file/d/1uAIx3rnNwrh1UNAMQCM0Fb9EkYIwKrgb/view?usp=sharing
Cheers!