I am playing on the newest snapshot, and keep finding bad generated things (in the nether) like flat generated terrain and right now a nether fortress. I were looking for a fortress a very long time. but today I decided to ride a strider and try to find one. after a couple minutes, I finally found a pillar from a brigde from a fortress. I thought that finally I could beat the game after a half year of playing on my world. I stacked up to the bridge, and suddenly I realized the fortress was only one chunk, did not have a blaze spawner and The game did not even count that as a fortress, because of I did not even get that advancement what says a terrible fortress. I hope you will fix this problem about the generating structures and terrain soon. Im not sure if you will care, but I do this so Minecraft will be better and released without bugs, not because I want to show how bad this is. I love Minecraft and that's why I ask to fix this thing.
To be clear, do you mean that there are sharp, straight 'borders' between old and new nether terrain? If so, this is unfortunately not fixable. This always happens whenever there is some kind of update that changes how terrain generates.