Some structures that were generated before 1.16 can't be located using /locate

There is a loss of data when opening a 1.15.2 world in a version at (or past) 1.16.

Steps to Reproduce:

1. Create a new world in a version before 1.16 with structures enabled
2. Enter the nether
3. Locate a fortress structure:

   /locate structure minecraft:fortress
   

    

4. Teleport to it using the command provided
5. Leave the world, and join in the latest version
6. Repeat step 3

The command will locate a fortress which is further away, despite the player standing on top of (or near) a fortress.