Material behaving differently on two different meshes

Hi. I am finishing a project I started a long time ago, probably 2.79/2.8.

I have this strange behavior where two meshes with the same material applied (no other materials on the slot menu) look completely different.

You can see there is a patch way darker than the rest.
I checked the normals:

Also, look different.

I am using the Sanctus library, but I doubt it is related to the shader.

Even when I take out the materials I can see something is off with the darker mesh:

I can notice some triangles on the darker white patch

Do you know what could it be?

Thank you

Overlapping geometry. Edit mode > M > by distance