Question strange 'reflection' on stretched polygons

Hello,
I have a suit model in Unity with a rig. When the arm is raised a strange sort of reflection appears in the arm-pit. I thought it was the mapping but if I change from tri-planar to UV this effect still happens.
And the image underneath is good.
This area is where the polygons are stretched a lot.
And we're using HDRP.

Does anybody have an idea what's causing this and/or how to fix it?

thanks!

 

@Flomotion70
Shiny? That could be a bad normals problem.
Put the model into blender or maya, recalculate or flip the normals, (or flip the normals, then re-flip them again), then import the model back into unity. Hopefully the problem will disappear.