i got a imported static mesh ( from packages of unreal market place):
and when i merge it to another sm file the uv map just disappeared!
Have you tried changing the UV channel that you are previewing. I merged three actors together and found that one of my channels was indeed blank. You can do this by using the drop down menu that is located on the far right of your tool bar.
no luck:( ,both UV channel are empty
however i did the same to a simple plane SM (from starter pack),and this time it works