Cannot label two mobile numbers as mobile on one contact
Works well for me. The first time I put in the second number, it made it into a "Home" number, but when I edited the contract again and changed it to "Mobile" again, then it works just like it's supposed to.

What version if iOS are you running? I'm running iOS 8.4 (I need to update to 8.4.1) on my iPhone 5S. The only thing that I can think of is a bug in the version of iOS that you're running.