Mannequin materials look bad after reimporting the mesh

Hi, if I export the Mannequin skeletal mesh in UE4 and theñ reimport it (with no changes in any external 3D package, just the same FBX file that was exported) the materials look bad, sort of distored or moved. This happens both when reimporting into the existing Mannequin asset and when importing into a new asset with another ñame. Material files and texture files haven’t been changed at all.

You can see the weird effect in the head of the Mannequin in the images: one image corresponds to the normal state before export-reimport and the other one shows how it looks afterwards (there are weird seams in the head and other parts of the body).

I’ve checked the triangle and vertex count and it is exactly the same before and after exporting and reimporting, and geometry seems to be apparently the same. Do you know how to avoid materials messing up in the Mannequin? I wanted to modify this mesh in a 3D package, but if materials mess up with no modification they will also mess up with external changes, so I want to fiñd a solution to this.

Thanks in advance,

Any ideas?