Was going to ask about what type of thread you are using.
It is possible for a new sensor to fail/have issues. To me it sounds like it didn't do this at first, now it is, and it still is.
This year I am doing a test with Superior So Fine as my regular bobbin piecing thread. Being a poly thread it really does have no lint, there is a noticeable difference. I can tell you that it is working very well, makes a fine strong seam that doesn't melt. The only (minor) problem I have is I bought a cone and it doesn't wind well on my vintage machine or my Sidewinder bobbin winder, but it does ok with the Bernina.
https://www.superiorthreads.com/thread/so-fine/c/60-205