UE4 splits vertices of skeletal meshes

I’m trying to import a character with a single animation from Blender v2.79 to UE4 4.17.1.

Everything seems to work fine until I try to play the animation: some vertices are moved and the character is deformed in strange ways.

The skeleton is fine, idem for the physics asset. The only thing broken is the animation that, of course, plays perfectly in Blender.

I tried to export both the skeleton and the animation back in Blender and noticed that the mesh is completely split. Please see the following screenshots.

This is the result of “inflating” the hand vertices. Everything is split.

This is the same command on the Blender original mesh:

As you can see the mesh inflates as expected.

I also tried with NON skeletal meshes and they behaved correctly.

Could someone help me understand what is happening?

I’m also looking for answer why UE4 split vertices on Skeletal mesh