It happens on most things but it’s worse on heavy fabric.
If I’m hemming it pulls the hem out of line even if tacked or pinned, so it ends up puckering or going wonky regardless of thread tension or stitch size. I’ve just hemmed a curtain and it’s an inch longer at one side with a good inch hanging over the end edge even though I took it slowly, I hadn’t pinned it because it was a simple turn over of the original hem so there was a straight line, which I thought I’d kept to.
It’s a basic singer machine, the foot looks slightly askew but it’s not loose, so could that be the issue?