You know that one where you can either go left or right. Left is the water boss and right is the library boss. Only if you go right, the door to progress the dungeon doesn’t open after killing the library boss. But if you kill the left water boss, the door does open.
I could have sworn it used to allow groups to go either way in the past.
What gives?
Probably a bug.
The “correct” path has always been water boss to library to dragon lady to sha. After spamming TW I have, for the first time ever, even learned a door opens behind the water boss. First group went to dragon boss and then final boss; I didn’t even think it would work without killing library boss, but it did.
Ah. So what you’re saying is that the door behind the water boss isn’t actually meant to open until you also kill the library boss? I see. Well I think it must be an old bug that never got fixed because the door behind the water boss has always allowed skipping the library boss as far as I know. I just thought the door behind the library boss would also allow a similar skip. But apparently one door is bugged but the other one isn’t. So everybody just uses the bug to bypass the library boss.
This has been a thing for a while now. I think it started way back sometime in Shadowlands or Dragonflight.
Basically Lorewalker Stonestep is now an unofficial optional boss.
Unofficial in the sense that he still appears as a “main objective” in the UI but isn’t actually one. Like that dragon in Darkheart Thicket. Really annoying when Blizzard does that.
I just assumed they did that intentionally because of how ‘slow’ Stonestep’s encounter is. But then again, Siege of Niuzao Temple exists so I guess it does make more sense that it’s a bug. 
Theoretically you can go left or right first, both have doors that open to the third boss.
But like others have said, the water boss is bugged and it opens the door immediately. I don’t think Blizzard intend to fix the bug either, not gunna see a whole lot of complaints against a MoP dungeon becoming even quicker.
We have always gone left first. We know there is an isolated boss to kick things off and get folks warmed up. Just how it is.