First I would not mirror the UVS both sides should use exactly the same uvs only mirror the object itself (mirror modifier would be the best way i think).
When you use Object->Mirror->… Try this: go into blender select the mirrored object after applying the mirror then ctrl-a apply scale after that edit mode shading/uvs (select every face) “recalculate” then export. Because what blender does with its mirroring it just uses -1 scale and thats what unreal seems to dislike. After applying the scale you will also see the issue in blender iteself.
If this doesent do it feel free to upload the .blend file and i’ll take a look at it