Does the interface set not work or is it just changing the case?
I've seen similar behavior with @removeable. No matter what you put, it changes it to a particular case\syntax and no other.
I think it has to do with what the hosts report back to the MC.
You can run a detailed diagnostics on the host to find the local interface charcteristics.
It will probably report the name of the Network Interface Set (Loopback/Loopback) not the value (Loopback/loopback).
Tom