Well despite their notorious lack of food requirements I do tend to feed her a lot so I suppose it could be that. Her abdomen is pretty plump. Maybe I'll drop the feeding down significantly (which is less fun but hey).Can't say what caused this obviously. However, I have experienced this with an adult Brachy. Not sure if my observation was the same as yours, ie its cause.
I don't know what caused mine to get a spot. It certainly wasn't because her abdomen was too huge and thus scraping along etc.
However, I speculate that perhaps the exoskeleton developed a weak spot for some reason.
All I did was slow down feeding and monitor. If a weak spot, I was concerned her abdomen would get worse with continued feeding.
She molted fine, and the spot is no longer a spot. New setae covered the area and it's almost healed. It's now a sliver instead of circular in pattern.