yup, got the same problem with mine, which is quite frustrating!
tried the same options as you did and yet no increased magazine.
got snapshit and skip rounds which don't make a lot of sense here, so I was hoping to have at least a larger magazine, but no..
I'm sure you've tried it (and even if this 'fixes' it it's still a glitch) but will it reload to a higher clip size if you have reserve ammo?
The thought occurred that there might be some conflict with the 'ammo on spawn' being somehow set at giving you a base clip size and that it might reload to a higher count.