I don't see this as being a leg problem at all. Your thumb is coming out early because you're not keeping the hand behind the ball (fingers at or below the equator) as it falls.
Consider your arm to be a pendulum system, with your shoulder as the pivot, your arm as the rod, and the ball as the bob.
The ball is going to fly off your thumb at some point in the swing. You determine that point by maintaining the ball at a certain angle to the pendulum. The ball is attached to the pendulum by way of your thumb. Therefore, the thumb can't be pointed parallel to the rod, or the bob will fall off.
So to sum up, the ball is attached to the pendulum by the thumb at an angle, which is determined partly by your thumb hole pitch and partly by the bend of your wrist.
As stated, your forward/downward swing starts with the hand behind the ball. However, by the time it's at the bottom of the swing, the hand is in a weak position. The wrist is relaxing a little relative to the arm, and so the ball is releasing itself. As it does so, your hand - which is not releasing from your arm - ends up on top of the ball.
Try an experiment: Use a credit card. Take a bowling stance with the card on your palm. Imitate the bowling pendulum swing. Draw back that card in a backswing and then forward in a forward swing, and throw it at a mark 5-6 feet away from you. You can hold onto it on the backswing using your thumb, but on the foreward swing, remove the thumb from the card. If you relax your hand/wrist position, the card will get dummped near your feet. You want to keep the wrist in a position that allows you to throw it farther.
I just tried this with my AARP
card and verified it. I can throw my AARP card across the living room, or dump it at my feet, based on my wrist position. You try it!
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current avatar is Gabby Hayes. Looks a lot like me!