I had the same type of problem with my Serpentine belt. The water pump started going bad (wobble) which caused the belt to eventually break. They replaced the pump and belt, but it broke within 200 hundred miles. They/I checked alignment, tensioner, and any thing else we could think of. New belt lasted about a hundred miles before it started cutting it into smaller belts.
It took about four or five belts before we found out that there was belt remnants left in the grooves of the pulleys. Not much but it was enough to have the belt get slightly off track. Once the pulleys were cleaned Really well, no more problem.