Nether Portals can generate encased in blocks

Steps to reproduce:

1. Create a world with the seed 800095968237666036
2. Teleport to X = 27, Y = 34, Z = -1619
3. Create a nether portal on the north wall, adjacent to the lava pit
4. Go through it

Expected Result
Portals should seek adjacent air blocks just like how portals generate above oceans with a platform.

Observed Result
Both sides of the portal are completely encased by basalt, blackstone, and gravel.