I just tried it fresh, this time in 4.15.1, and had the same problem occur. Testing with 3DS Max 2015, using fbx2014
I made a cube, set up uv 1. exported and brought into Unreal. Uvs correct. I then go back to my cube in max, add another UV channel, do something with it, and export over the original fbx. I hit reimport in Unreal, and the new uv channel is missing. As proof it’s the reimporter, I then imported that same cube fbx fresh to a different folder in Unreal, and it contains all the uv channels correctly.