The bug

Nether fortresses often spawn entirely surrounded by netherrack.

How to reproduce

1. Create a new world with the following seed

   -1192776587582434135
   

2. Go to this location, in spectator

   /execute in minecraft:the_nether run tp @s 4798.85 65.92 -653.98 -166.21 35.55 

Note: this doesn't only happen in the above example, it is quite common