One of my 4 month old budgies is balding around his vent, he had this when we first picked him up from the store. His stool looks normal in consistency, nothing that would indicate diarrhea. We never see him or his cage mate pulling feathers out of the area. We purchased them from bird store that sells birds from local breeders. He was in a cage with multiple other budgies. He also seems to have a lump in the vent area as indicated in the picture.
I have read on the internet one such possibility could be other budgie cage mates picking out the feathers.