Hi there!
I’m working on a low poly hand using Blender and UE4 4.27 for a mobile VR app.
In the hand “closing” animation, I have a corrective shape key to make it look better when the animation is completed. In blender everything look as intended. I have set all the edges as sharp and activated auto smooth with the degrees at 0, and I even save a custom split normals data.
When exporting as an FBX, under the geometry panel, smoothing is set to face and “Tangent Space” is checked.
In the UE4 import settings, the normal import method is set to “Import Normals and Tangents”.
The result in UE4, is that the corrective shape key works fine, the animation work fine too, but the smoothing groups look wrong when the shape key (morph target) activate and the bones blend. It’s like the morph target and the movement of the bones distort the smoothing groups 
, and instead of a flat shading, the result is some kind of distortion in the hand and weird smooth areas where they are not intended.
I have tried different settings combinations, check and uncheck all the options for the smoothing groups… I’ve tried pretty much everything I could do… But I didn’t find a workaround for the problem 
As It’s for a mobile VR app, I don’t want to split all the faces to get that low poly style. If anyone knows any fix for this problem, it would be highly appreciated 
!
Thanks for your time!