I have a working theory, not fully proven, but close enough: My machine won't let me FMQ entirely backwards. It breaks the thread after a few inches.
I wanted to just do a wiggly stitch on a border, and knew the quilt was easier to handle if the bulk of it was moving towards me. So I thought, "Why not just FMQ in a gentle meander, but backwards?" I soon found I couldn't stitch more than a few inches without lots of bobbin thread gathering up, then the top thread breaks. After lots of testing and re-threading, etc, I finally got it working correctly. I think. It's sitting there waiting for me to get back to. At first I thought a grand kid had gotten in there and twisted the tension knobs while I wasn't looking, when they were here for the eclipse on Monday. He's 6 and pretty adventurous, to say the least. But then I felt bad for thinking he did something. Fortunately I didn't go accusing anyone. That never turns out well, haha. So I'm sticking with the "No backing up" theory for now.