I encountered a nether fortress where blocks of a basalt pillar had replaced the nether bricks.
How to reproduce:
1. Create a world with the seed 2277298773346079674
2. Go into the nether and teleport yourself to -82 67 -85
A basalt pillar has generated inside of the nether fortress, replacing some of its nether bricks.
I am aware that this was reported before in MC-171553, but it seems to occur despite it being reported as resolved.