I have the 6700p and use the HP plate and foot all the time. I do use the accufeed foot for HP and my machine doesn’t do what you are describing. It sews beautifully for piecing. I use it to attach binding and it gives me no trouble with the diagonal seams from joining the binding.
i would take it back to the dealer and show them what’s happening. There’s no way you should have to put up with that. Hope you get it sorted soon.