Unclear if this is a mod compatibility or not, this was reported with Twilight Forest installed.
Only reporting from a guess as to what may have caused it; somehow the teleporter sent the player to the equivalent portal location in the Overworld when they were at the bottom of the Aether, which being ~y=0 would send the player into caves, instead of sending them to the top of the Overworld as intended. Could also be a weird instance of a teleporter sending an entity to 0,0,0 as I've seen in the past from client/server teleportation issues.

Considering Twilight Forest is also in pretty early beta for 1.18 it seems and we plan to rewrite/cleanup our portal code, I'm not sure how relevant of an issue this is to worry about right now.