I’m a bit stumped to be honest.
The only time I’ve experienced repeated jumping is when I had the QR spring too loose, and so it wasn’t holding the dog against the drive screw tightly enough… How did you fit it? Do you have any photos of it off and on the vise?
Ahhhh… this just sprang to mind: the QR rod needs to on the right way. The wrong way puts it slightly to the left (i.e. slightly out of the slot under the dog), so making it more likely to jump out under load. Also, if the QR rod is not screwed down at the right angle (i.e. deflecting away from the dog slightly) that might make it jump out.
Either the bar is jumping out of the dog, or the dog is jumping off the drive screw.
I would get under the vise and have a look at the bar and the dog while someone else is operating it to try narrow down what is going on.