I’ve just added the following to the OP:
Frostwyrm Lair
- Once players have cleared all four wings and entered Frostwyrm Lair there is no teleporter or other method to return to the lower levels of Naxxramas.
- This includes leaving the instance and returning, as you will be ported directly to Frostwyrm lair upon Entering Naxxramas once the portal has become activated.
Edit: This has been removed from the Not A Bug list. Thank you!