Worked this out today. 15.9gn JSBs are not long enough to interfere with the o ring. So you drop them in the mag and the ring stops it falling through but doesn’t grip the pellet, so if you tip the mag back with a pellet in the open hole it’s falls out.
Design fault? Not really as the o ring has to be where it is so it doesn’t interfere with the indexing mech. Also the mag pellet hole has to be long enough to accept all lengths of pellet available in the caliber.