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:
Build a nether portal and enter it.
Destroy the nether side of the portal and go above the ceiling of the nether.
Build a portal and go through it.
Enter the overworld portal again. A new portal should be created below the ceiling, but instead you end up at the one above.
Thank you for your report!
However, this issue is a Duplicate of MC-160677.
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