Thanks to everyone who had suggestions about what was causing the biasing.
Most of you seemed to think that it was the yarn. Single ply yarn simply knits up that way. It’s certainly nice to blame my materials and not my technique.
It does mean I’ll go back to the drawing board for the cushion pattern though. I have a lot of this single ply bulky yarn and it would be a shame not to use it, so I’m thinking of maybe incorporating some every decreasing squares in a subtly twisted stitch. My brain can be working on this whilst I’m finishing up the hug monster order.