Cracks in models on materials edges

For some reason my latest sketchfab uploads have a strange issue with vertices on materials edges disconnecting from each other, causing cracks to appear, despite the model being a single mesh without any holes in it. I’ve tried using different versions of blender to save the file and even converted it to other formats. Rescaling the model didn’t help and I even asked a friend to upload the same model from his PC and it still had that problem.

The cracks are most visible on the character’s head, as I’ve used 3 different materials there.
I’ve also added a screenshot from blender where it’s visible that model doesn’t have any cracks on it.

This is very annoying, as I am trying to use my sketchfab profile in job applications. Please help!

@waleguene could this be the result of geometry compression?