Bavaria 36 2002
Ours used to jump out of gear. It was caused because the lever became loose and the only thing that keeps it engaged is a plastic bump fitting into the back of the lever.
Until I could do anything about it we found that one of my wife’s hair toggles wrapped around a bar and hooked onto the end of the lever solved it!!
I don’t think tightening up the belt mechanism will have any effect on the lever jumping out of gear.
Serendipity.