Actually, I think it probably belongs here. Have the health side of things checked first then if no clues there, then look at behavior. To me, the vet visit should be the first step so it’s great you have that lined up.

Not sure why you need to switch his hay if he’s eating timothy.

What type of pellets are you feeding him?

Well, normally it’s the pellets or greens that can cause a poopy butt not the hay.

It sounds like he needs to eat more hay.

Do you measure his pellets?
What kind of vegetables are you feeding him?

While he has the diarrhea I would not give him any fresh greens but make him eat just timothy hay and add a little oat bran to his oxbow pellets. I would cut out the green beans completely from his diet.

How much pellets do you give him?

I would just give him a 1/3 cup a day (maybe half in the morning and half at night). Also I don’t recommend the oat bran stuff – this is full of carbs which can also cause poopy butt. Better to stick to the basics. He probably won’t be happy about the pellets being cut back but it might help slim him down as well.

Is he a good hay eater?

I was thinking of feeding a small quantity of pure oat bran, replacing some of the pellets (Oxbow organics e.g. contain starches in the form of barley) while the rabbit is suffering from the diarrhea. Oat bran is lower in carbs than actual oats, plus much of the carb content in oat bran is soluble fibre. Both rolled oats and its bran have been used in small quantity, in combination with hay, to cure diarrhea in rabbits. My mini rex came to me with diarrhea and my vet suggested to feed just hay, oats or oat bran and salt from a wheel for five days after we had first tried leaving out different green items, and switching pellets. It did the trick.
You can easily fatten up a bunny by giving grains, or overfeed in starches but the combination of oats and hay has a place in the nutrition of a sick rabbit; mostly because they are not as processed as even the finest pellet.

Sassy, if he’s already on the mend, maybe he just ate too much greens which are high in water.
Monitor his progress, though. I did the oats and hay when my bun wouldn’t improve with anything else tried.
I am wondering, too, if Flopsy got a little stressed out from his roaming adventure. some animals are sensitive that way, you know, a little stress can upset their tums. So glad he is getting better! This rabbit has kept you on your toes!