morph target turns out gray color


I am working on a project, which requires a lot of morph targets for facial and corrective blendshape.
I made some of the morph targets with shape editor in Maya.
It looks absolutely fine in Maya. However, when I export it to unreal, there are gray spots here and there on the surface.
I tried to work on a few weeks for it. But I couldn’t solve the problem so far.
Is there anyone who has this problem before? or solve this?