I have had that issue with aging belts where the joint is losing it's adhesion capabilities. Get fresh belts from a reputable supplier. Be careful to not grind a sharp or narrow edge because this may catch on that splice separating it from the backing.
Tru Grit has been my go to for abrasives. This is their 1x72" list:
https://trugrit.com/product-category...type_length=or