That will be this adaptor.
The adaptor cannot be at fault as its only a piece of tubing with a tool fitting on. There is nothing to be faulty.
It will be more inclined to do it if the cyclone and/or the filters have not been maintained. A slightly blocked filter, a slightly blocked cyclone and maybe a sticky entrance valve and then "blocking" it with a vacuum bag will make any V6 pulse. Like Ryevac says, it thinks it is blocked - which it essentially is.
It can be minimised a bit though.......
Make sure your filter is clean (washing machine) and dry (or new), and the flappy valve at the entrance near the front of the bin moves free and the small cyclone holes aren't clogged up with debris. Use the adaptor allowing a little air to flow past - ie, not so tight. That way maybe 80% bag suction, 20% escaping suction. This way you are not entirely blocking the machine. A bit of trial and error and you'll soon get the hang of it.
That said, the V6 doesn't have much suction in the first place so dont expect the earth. Like Ryevac says, a proper vacuum is better really. Yes that adaptor fits the V6 but it isn't the adaptors fault that your V6 isn't up to doing with the vacuum bag what you want it to do.