I have seen this reported on the forum a couple of times, with no obvious root cause, so it might just be a random quality issue with the belt (the heatwave we got in Europe two weeks ago may not have helped indeed)
I have come to think of belts as another consumable item, two years ago I got a roll of steel-core belts (like those), swapped the three belts on my SO3 a few times, and have not yet used up the whole roll.
n.b.: in case you missed it, you may be interested in trying @LiamN’s methods for precise belt tensioning when you install the new ones