If it was CD10 for mild / moderate, I could understand the FDA maybe wanting to see some more data there.. I don't think they want any more slots filled in M/M though; there are quite a few already, and the patient population generally doesn't exhibit a dire need.
But you're right, OLE under CD12 makes no sense to me. Unless CD16 is going to be looking at more specific, narrower cohorts than CD12, and that would allow the rest of severe/critical access But then there's still overlap.