The coating shouldn't come off, so they're right. Send the brush in. I bet they will strip down your AB, replace the body and reship to you.
No, I haven't seen this problem before, but it IS a coating over the brass, and things do happen. Just like release molds repel paint, I suspect the cup hadn't been properly cleaned before chroming.
They're doing the right thing, which fills me with satisfaction and a feeling of well being. Considering how many of their airbrushes I own, and how much I recommend them, it's a bit of a relief!