Leg came off again. I took a close look, and it appears to be a tolerance issue (go figure).
The first issue is that the plastic is more than half of the ball.
This means that in order to maintain integrity, the socket has to keep the metal half encapsulated, which is less than half of the circumference. If the socket gets too wide, and the shallow equator is exposed, the ball can separate.
Low and behold, the plastic on my right hip is too large for the socket causing the socket to be wider than intended.
No ball inside:
Ball inside:
The screw sits behind the ball, so there is no way to tighten down the socket if the ball doesn't fit.
I have modeled a slightly smaller ball cap, and printed it out. I will post progress.