I was midway through a bountiful delve (Dread Pit) when the servers crashed.
To my surprise, when I finally logged back in, my progress was saved and I was still in the delve. I finished it with 4 lives remaining and there was no bountiful chest at the end, just 2 green chests.
edit: another detail is that it remained bountiful on the map after completing the bugged out version of it. I was able to then do it as bountiful and get the chest. In the bugged out version I was also getting no Brann xp, so I guess it converted into a normal delve when the server crashed.