I don't know anything about the Panther belt other than the name.
I did happen when throttle was applied during 1-2 shift. I usually shift hard, but this particular day was a very easy shift and then that happened.
The Gates belt popped on a hard 2-3 shift a couple days ago.
It's an '07 XB9R with Jardine can, K&N filter and custom tuning. Unless a PO did anything else internally, it's still stock.