I don't know anything about it but I do believe one kitten in the litter can suffer from it while the others are healthy. Maybe with her being so much younger things didn't develop fully in time. I wonder if the spay triggered things and that is why the previous foster thought she was healthy. I wish Laura was here to answer your questions.

Hopefully this is just a case of a slow recovery from being spayed. I personally haven't heard of it but I haven't been around a lot spayed kittens and I bet it's possible. Can you call the vet that spayed her and ask what they think?