Not sure if this bug affects the future version, but my world has Experimental Gameplay turned on if that matters. I turned on Experimental Gameplay long after I first went to the Nether. Basically, the fortress went into netherrack, and therefore had to create air blocks to be accessible. The problem is, there was fire above the air blocks, and the fire blocks weren't removed. The outcome is what you can see in this clip I made. If you put netherrack under the invisible fire, it becomes visible again and you can put out the fire.