Likely this is because B9 is overriding onDataChanged in char_weapon without calling previous copies via super.onDataChanged.
I don't know this for sure, but that's the most likely thing I can think of. If I'm correct, the hiding of the ammo checkboxes and linking of the ammo field when you link ammo should also not be working.
If overriding this is required for the features of that extension (since of course it's usually best to call the prior function when you override it), it shouldn't be hard to add some compatibility code to the replaced function based on this.

