For Nether fortresses, the building element with the id:NeMT, i.e. the one with the spawner.
The bounding box is one block too low, which is why the top 3 fences are not in the bounding box.
In the screenshots you can see the coordinates of the bounding box and then what the structure in question looks like.
Example:
Seed:-3960378471346730310
Coords: -142 80 86

Steps to Reproduce:
Create a world and enter the Nether
Use an NBT editor and search for the structure data of a Nether fortress
Find a structure part with the id:NeMT, i.e. the one with the spawner.
The int array BB now provides the corner coordinates of the bounding box

Observed Behavior:
The bounding box is 1 block too small

Expected Behavior:
The three upper fences of the structure are inside the bounding box