I have 20k on my seats and have a car seat in the back row and haven't seen anything like this on my truck. I would guess that it is a fault in the material.
That's awesome you were able to find one in the meantime. I had that same Jeep in 1994 myself and if the new Bronco your grandkids have is anything like it, I am sure they are having a blast!
For real, the seats look awesome. I have the factory MGV seats and I have really liked them so far, but these look like the materials would be even softer/more comfortable. But WHERE DID YOU GET THAT AMAZING CUPHOLDER!?