Ive had the same problem for a while now BUT i think i’ve found a fix.
Im a Maya user, so when im Exporting an FBX for a skinned asset, be sure to have triangulated on and smoothing groups. ( as mentioned above posts ) but in Unreal when the import Box comes up look for " Normal Import Method " - change it to " Import Normals" ( which will read the normals from the imported FBX )
I think by default its set to " Compute Normals " which will let Unreal make its own Normals up ( which is why they look ■■■■ on import )
This has sorted my problems, hopefully it will help you too.
GoodLuck!