You are paying a premium price for a M154621 belt. Know your dealer likes to bump the price even more than JD list. OEM Deere belts are physically beefier than the aftermarkets so stick with them. I have had a couple of people that had the same issue. Every time the deck is engaged there is shock load on the belt. If the belt gets caught up into a guide, the torque applied at the clutch will snap the belt. Real excessive forward pitch can get a belt pinched. Think of a belt drive like this as pushing the belt through the maze of pulleys rather than being pulled through by the clutch and you will look at this a different way.
Two things within an operator's control is to engage the deck at 1/2 to 2/3 Throttle and to use the mow in reverse feature rather than keep engaging/ disengaging the deck. AT 500 hours, you do likely need to freshen the bearings and likely at least the vee idler in the back if not all.
One other item on these that can occur is that one adjuster at the rear of the deck runs close to the belt. I've only seen one that actually could hit the belt though.