[Harry_p]

Looks like paint too thick, and / or not enough drying time between layers. It looks like the paint itself has shrunk and cracked rather than the repair underneath failing.

 

[Harry_p]

You can see the edges of the paint have sunk and rounded as its shrunk down.

Sure, the repair technique might not be ideal, but if it had been the repair underneath failing the cracks would have a sharp, hard edge to the where the filler had broken off the bumper, and would usually appear around the edge of the repair first.