I located a nether fortress with /locate because i wanted to find another one. I flew there with my Elytra and wanted to farm some wither skeleton skulls. But even on hard difficulty there were no mobs spawning inside this fortress. My original fortress still has mobs spawning in it.
If anyone wants to check the world seed is 2581424116973640431 and the coordinates the /locate command gave me are 784, ~, 912.