I was having the exact same problem - my assets were low poly and not smooth when I imported them in to Unreal. This thread ended a 6 hour search, so thank you! This is what I did:
Delete ‘Non-Deformer History’
Modify->Convert->Smooth Mesh Preview to polygons.
On export choose a preset in the menu - make sure smooth in unticked - this is a bug or something
On import import normals and tangents
Thank you to everyone here, I hope other people will find this thread as well.