I haven't looked to see what you did to fix it in nUI 6 but I just added PetActionBarFrame to my temp movers addon and it works every time.
It will suffice until you have more of the fancy stuff in version 6. It's fine for testing but I just can't play using it in its current state.
I'll try and code it to work as a more flexible mover plugin for nUI whereby people can temporarily add frames they wish nUI to track and move until they're added to nUI proper. It's just a simple one at the moment and not remotely user friendly
Here's a screenshot showing the non working pet bar holder and the working pet bar holder on the same screen.