Yes, I know about the QR bar. The hole for the screw is slightly offset. Guess how I found out 🙂
The QR spring is tight, really tight. It is hard work to squeeze the QR handle. It is holding the half nut on the drive screw just fine when the jaws are wide open. However, that does not seem to be strong enough for the first 4″ or so.
It is definitely the dog (or half nut as I call it) that is jumping off the drive screw – already checked that.