User Changed Device Name

In our design the user may change the device name with a custom name, up to 20 characters.

This only seems to work if I "reserve" the 20 characters in the GUI dialog for device name.  For instance if the name in the gui is set to "test" and I programmatically change it to "abcdefghij" the name is saved as "abcd".  If I save the name as "testabcdefghij" in the GUI then I can change the name to anything that length or shorter.  I hope that makes sense.

Is this normal?  Seems dodgy.

Rich