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!