For some reason the forum doesn't let me upload pics so I can share my events here. If I add a link to the pic, the post does not get published
The logic I use is very simple, the flipper has a 60 degree limited revolute joint. When the flipper is touched, I apply an upward impulse on its edge. Then I every tick I apply a smaller downward impulse to help it go back. That's all. And it works great most of the time.
Setting the bullet behaviour didn't change the leaking through, but on the flipper it breaks the revolute joint.