it doesn't happen on the orginal, according to the sneak anims I just inspected.
It looks as if its a rigging error- mesh tearing or ripping. Not exactly sure how that was introduced. best solution is to open the nif, and edit the vert weights around the edge of that inner mesh, to again match the coresponding verts weights of the outer. could do the copy/pasting weights as individual verts by hand, as its only about 20 verts or so. a small bit of tedium. or import the weights all in one go.
or you could be lazy, seek a subpar result- add a nistencilproperty to that nitrishape block via nifskope. at least you would'nt be able to see though it then. it's not optimal, but it could be a quick fix