Yep, we're aware of it, and I think I've found the source.
You're actually still able to change ammotypes, so you can fire any and all of the available rockets.
...you'll just be displaying, and using up, your whitephos ammo to do so, because the projectile and the ammo are not actually directly linked.
In fact, I suspect that once your whitephos ammo is all gone, you'll start using up rocket ammo.
This will be fixed, anyway.
(and yes, it has been mentioned before, but don't worry)