I had the same issue before, here's the solution I figured out.

1. Go to UV > UV set Editor.
2. Rename "all the object's you want to combine" to the "same" name. (select the object one by one and Rename all to the same name.)
3. Combine all the object, should be good to go!

So the trick is sometimes( no idea why) Maya create a different Uv set, so when you combined it, it will show in different sets. So it's not disappear, just in different Sets.

Hope that works for you!