When I created a portal on the bedrock ceiling of the nether, the game counted it as a valid portal. Even though it was above y=128, going through the overworld portal still teleported me to the one above the ceiling.

Steps to recreate:

1. Build a nether portal and enter it.
2. Destroy the nether side of the portal and go above the ceiling of the nether.
3. Build a portal and go through it.
4. Enter the overworld portal again. A new portal should be created below the ceiling, but instead you end up at the one above.