Ive done this many times before and this has never happened. Im importing flat faces like this for some tailights:
https://gyazo.com/5b4417b7ff52cc1f0a320aeb81131c58
As you can see, its nothing complicated with proper normals.
Once imported this is what I get from looking at both sides:
https://gyazo.com/20c65c1fa82913e55a4946a387f6e39f
Funny thing is I uploaded a mesh just like this for headlights and it worked.