I also found that scaling groups cause odd reactions.
the current workaround I use is to model everything in inches as cm, so if a Door is supposed to be 210 cm high then i make it 210 inches, but set model units back to CM before export. when imported to UE4 the bug brings it back to 210 units (cm)
its not ideal, but it works.
I see on the tracker they pushed it over to 4.22, so ts not going to be fixed soon. i was actually waiting for the fix before pushing out a lot of work, but I will have to work with the workaround for now