Precipice → Ancient Cavern Time Warp Portal Disappears if you go back into the Precipice from the Ancient Cavern. Had to redo the zone in order to get it to show back up.
Reproduction steps:
Enter The Precipice
Kill Boss
Enter portal that appears and end up in the Ancient Cavern zone
Enter the portal at the beginning of the Ancient Cavern zone and end up back in The Precipice
See that Portal to go to the Ancient Cavern is gone