It has been like this for some time now. I have done some extensive testing and the damage is 2d8+11 Acid damage per round. (since lowest damage I ever got was 13, and highest 27)
The numbers seem a bit too specific for there to be a mistake. Or maybe there is an extra "0" in there, so +1 became +11
The area more or less requires either a Cleric or a Healer Cleric + lots of Healing Kits, or LOTS of healing kits and HP to tank through the damage.
EDIT: To add to the excellent post of AstralUniverse above:
-With ESF: Abjur, Endure Elements absorbs up to 45 damage, meaning you can keep it up on 2-3 people at least before the first one breaks. Assuming the rain doesn't start to double-hit. Still need Regenerate though for the damage to be manage-able.
-Shapechange: Balor also negates 20/- Acid damage + lots of Temporary HP, but means you will mostly be standing there, as the Balor's claw attacks aren't very threatening.
-Dominated/Summoned Fiends are (mostly) immune to Acid damage and can navigate in the environment, reducing the need to heal as many targets.
Truth be told, I do quite like the area as it is now, whether that is intentional or not. I like the concept that there exist places on the server that are "hard" and require some specific classes, or clever tactics to help you get forward. Not everything should be accessible to every character solo across the module. Like my low STR character Sorcerer can not manage many "climb checks" without a friend, but my friend would not survive... say, Mind Flayers without my Sorcerer's Mind Blank.
Anyhow, here are a few thoughts:
- Underdarker's route into Nine Hells is now extensively harder. Not necessarily a bad thing, if the design would like everyone to funnel down "from top (Avernus) to bottom (Minauros)". A "shortcut" straight to 3rd layer exists (the old route) but requires one daring the weather for longer (the massive damage acid rain). Perhaps add a teleporter from Freth Spire to Dis if you still want people to be able to progress layers in an alternative order.
-If Minauros is intended as an "epic area" and the acid rain is intentional, a few "caves" or "huts" in where characters may take shelter from the downpour would create opportunities to rest, heal up and ward again. Potentially rushing through areas, or sending one character to "lure" the devils to the cave in where the rest of the party are waiting.
-Maybe a Fourth level is being planned? To await behind the wall of acid rain.