I see this happen at some point with all of our bred-by Chihuahuas, no matter how old they are. When we have a litter of puppies, the moment we set up the Iris pen in the living room, they go back. They go back to the days of their own puppyhood and want to feel that comfort and safety those four plastic walls gave them way back when. Sure, the never ending supply of puppy food is nice, but I have seen them cuddle up in the doggy bed and really struggle with whether or not they should come out.
Bella is 9 months old, what I would call a puppy teenager. We opened the door to the Iris pen and she almost immediately jumped from her big-girl doggy bed right over into the pen. The doors remained open the entire time the puppies were out playing, but Bella wasn’t coming out. She stayed in there until we had to actually pick her up and take her out. She licked and chewed on the bars like the puppies usually do, chewed on the puppy teething toys and even fluffed the doggy bed, like she knew she was going to stay awhile.