I hope this gets seen as its and ‘oldish’ report but here is an FBX that exhibits this behaviour 100% of the time. OS X, UE 4.2.1. FBX file is from Cinema4D.
Import the FBX and make sure ‘Combine Meshes’ is OFF. You will get a number of very similar meshes that have different UV Maps.
Highlight them all and click Reimport, every one becomes a combined mesh with all the UV’s combined. This also happens reimporting a single one.