When clearing all 4 wings of Naxx it immediately teleports you to Sapphiron’s room when clicking on the end teleportation stone at whichever wing you cleared last.
If you wipe and zone back into Naxx, after clearing all the wings, then it immediately teleports you into Sapphiron’s room as well.
Which makes the rest of the instance in-accessible to clear the other trash / collect frozen runes.
When we zone into naxx, we zone onto the TOP platform rather than the BOTTOM platform inside the center area / entrance. Also when teleporting back from end of wing (clicking the teleporters) as above. This causes instant teleport to sapphiron’s lair because the entrance to sapphiron’s lair is the top platform.
This appears to be an oversight in the current classic version - it might even match the reference client - but is at least not the behavior on shadowlands in the northrend/lvl80 version of the instance, nor was the behavior in wotlk. Zoning into Naxx should put you at the bottom platform.
(Note: it is understandable if the current behavior matches the reference client; the wotlk behavior is just the better, practical (and common-sense) design)
I can confirm this bug as well. Even after Kel’Thuzad was dead, we couldn’t get back down stairs. I seem to remember zoning in at the bottom on the green pad, but having to walk up to the blue pad to get to Sapphiron.
Hopefully Blizzard fixes this. We were pushing for world progression and just left all the runes and extra trash lol
I don’t think this is a bug, just how it always worked? If you want frozen runes you need to grab them before killing the first 13.
The WOTLK Naxxramas changed this.
I do not believe this is working as intended. Due to the fact that when you clear the first 3 wings it teleports you to the lower platform. As you can see from our first clear video - starting from approx 26:42 - you can see our MT click on the teleportation stone and get brought to the lower platform.
Sapphiron’s room was only accessible if you went to the upper portion and walked into the center after clearing the 4 wings.
This is currently working as intended. Once you clear all 4 wings, the teleporter to Frostwyrm Lair activates and once you take that teleport, its a one-way trip. This matches the behavior in Original WoW.
It’s definitely advisable that if you want to grab any frozen runes on the walls, or do any other exploration of the dungeon after clearing each wing, you should do that before entering Frostwyrm Lair.
I’ve gone ahead and added this to the Not A Bug list… for now.
Edit: Turns out “for now” wasn’t that long after all… 
We’ve gone ahead and hotfixed this to allow you access the lower levels once again by re-entering the raid instance. Pazorax posted more about this here, and I’ll be removing this from the Not A Bug list now. Thanks everyone!
That is a terrible set up though… even if its consistent with the vanilla client it should be hotfixed. This is the type of thing that would have come up and been fixed in vanilla except that hardly any guilds were doing naxx. And I think you’ve also glazed over the fact that they are saying the end of wing portal takes you there… not even stepping onto the portal in the center of the raid.
Edit: For clarity: In your post you say ‘Once you take the teleport to frostwyrm lair its a one way trip’ and the complaint being discussed is that they are NOT taking the portal to frostwyrm, they are clicking the portal at the end of the wing they cleared last… Which even if ‘consistent with the original client’ is clearly not logical behavior.
It seems like being able to navigate the instance to your needs or preference, and even zone out is something that should be doable - we couldn’t even step out to summon after running back from a wipe…
Just a heads up to everyone. When you clear your 4th wing, if you click the portal behind the boss it WILL send you up to Sapphiron. So if you want to stay, you need to do an about-face and walk out of the last wing you are in to move to other wings to collect or farm whatever trash you want.
Thank you for addressing this Aggrend and clarifying the intention.