I have a thread cutter and a 'fix' stitch. If I use the thread cutter, the fix stitch is automatically engaged when I start to sew again. I think the idea was you cut the threads, so the new line of sewing needs to be secured so it does the fix, which does create a little knot. When I use the cutter, the fix button lights up, I can push the button to turn if off. but I never remember. This might be what's happening with yours or as mentioned, you are not pulling the bobbin thread to the top. As far as I'm concerned it's more trouble than it's worth.