I try to import one of my fbx from blender to ue4 25 but it lost some faces.In blender it looks great (the face orientation is also good) but in ue4 looks like this.
https://prnt.sc/v9jb1u
https://prnt.sc/v9jbhk
Are you sure all normals are facing in the right direction?
In blender you could check this by enabling “Backface Culling”