Nether Fortress generated a single slice of a hallway.

Did you have a previous world with the same name?

This could be a chunk glitch where some chunks are not deleted when deleting a world, and creating a new world with the same name will have those unremoved chunks.

It is more noticeable when you make a superflat world, delete it, then create a default world with the same name. with a bit of searching, you can find left over superflat chunks.

This is my 1.8 testing world and was called "1_8 snapshot". The Nether Fortress was not on a chunk boarder.

Was the world ever reverted to a previous version by accident? If not then I'm out of ideas.

Cannot confirm.
In a new generated 1.8 world is a complete fortress at the given coordinates.
Please recreate that world and check the coordinates. Maybe you had the world created / opened on some snapshots and destroyed the world ?

Ticket resolved as incomplete, because no answer in a reasonable amount of time (1+ year), if it still happens, please update the ticket.