Call me crazy, but I think if you hold the right flipper while the ball is being held by the prison magnet, it somehow 'charges' the magnet, and you are definitely not getting a SDTM. Try it. I've played on a Pro on location, approx 150 games. At first, I was having trouble with the magnet, and STDM balls. I can't take credit for this possible solution, as it was my girlfriend who noticed this.
By the way, she is simply a supportive Pin-Wife, and really seems to like TWD. She has asked me to go out and play. Most impressive.
Cheers from Canada, gents.