I think it is a bug. I play with autonomy off, so my sims cannot just decide to do something else. What happens in my game is they put the toddler on the potty, then after one or two seconds the interaction gets canceled for the adult while the toddler continues doing its thing. Something else drawing my sims' attention cannot be the cause for this.
My only question is, does the toddler still benefit from doubled learning speed from getting helped, since the adult started the interaction? If yes, then I don't mind this bug at all.