Generally true watery diarrhoea is considered an emergency situation, since they can dehydrate very quickly. I’m guessing the vet was ok with it because of him still having normal poos and eating/drinking. If he has another episode of diarrhoea I would be taking him in.
Rabbits should have access to hay at all times, so leaving hay in overnight is a good thing. What sort of hay are you feeding? Is it a grass hay, or is it lucerne/alfalfa hay? If it’s alfalfa then it’s too rich for him. Alfalfa is a legume hay and very high in protein and calcium and is too rich for rabbits. If that’s the case switch to a grass hay. If he’s already eating a grass hay then I’m not sure of the cause. Grass hay should not cause poopy butt like that, it should actually firm up poops and increase their size because of the increase in fibre. Long term I’d be looking at reducing his pellet amount and increasing hay consumption.
Keep a close eye, like I said if it happens again, it’s probably worth a vet visit.