I lost 1/2 of the top bearing once. It happened when I was working to convert the LD to a SL unit. I lifted the entire luff assembly and when I lowered it I heard something hit the deck and then a splash. It took a little while to figure out what had happened. Here is the final verdict: Properly installed, the top of the foil assembly should be about 1-3/4" below the swage at the top of the headstay wire. While not snug, the hole in the top bearing fits the forestay wire, but it is only slightly smaller in diameter than the swage fitting just above it. When I lifted the luff/drum assembly, the top bearing was jammed onto the swagged portion of the fitting that connected the headstay to the mast. When I pulled the luff/drum downward, the top bearing remained stuck on the swage and pulled out of the foil extrusion. The small screws and plugs holding the bearing halves into the foil weren't meant to handle this type of loading.
I purchased a new bearing assembly and installed it in the usual way and have not had any problems since. However I am very careful to monitor any luff upward movement if I am working on the furler.
IMO, it would be pretty hard for the bearing to work its way upward in normal use. I see you are from the colder region. Are you stepping the mast for winter storage? Does it happen during this process or when you re-step the mast in the spring? Could be you are experiencing the same situation I did.
Good luck with finding the cause!
