The crimson forest can overlay the nether fortress with the "trees" being grown completely inside of the hallways blocking the way. The "trees" do not break the fortress blocks but grow through them
Linked issues
is duplicated by 3
Comments 2
This is working as intended per MC-171328.
it's a biome related much like mineshaft Spawning on ravines, the game is generating unexplored chunks in which they would spawn. the game generates random you could potentially see some weird things but it's normal it's the game generator