Morph target sliders flipping normals in 4.15

I have a simple head mesh, with two blend shapes exported from maya 2015 and imported as morph targets. When i adjust the morph target sliders, Half the faces on the mesh will ‘pop’ to black, as if the normals are flipping. This is not seen in maya, just in UE. i have tried every combination of export and import settings, and i see the same thing.

What could this be?

thanks!