Here we can clearly see the defect: the strap hole is worn-out, so the strap is not fixed anymore and can pull through the sole.

From the bottom side, we can see that the foam hole has become too large to keep the strap "stopper" in place.

Preparation: We're going to create a plastic washer to keep the strap in place, and for this it's useful to have a guide to trace around. We're using a bottle lid, but a coin or anything larger than the hole will work too.