Joe, so you're saying that the top of the ball doesn't touch anything if I read you correctly. If that's the case, sounds like the key is now moving up the about 5/8" because there is all kinds of space then the balls retract.
The standard hitch for a ball is a actual cup so the key could not open. Where's they pressure pulling up with pressure on the balls, the balls may be raising the key which would normally be captured by a solid cup.
Typically, this ball wouldn't see these type of forces only front to back and the balls do very little other than keeping the shank of the ball in the sleeve.