Zoom out that map, then change the version selection to pre-1.16 and you'll see the fortresses did cluster in strips parallel to the z axis before, but they don't do that anymore. You'll see also that nether fortresses aren't generated as closely together as before; they are now less common.
Hi. I had to undo your edit. Since 1.16, nether fortresses generate randomly all over the place.
You can check it out here: https://www.chunkbase.com/apps/nether-fortress-finder#4893920111
Zoom out that map, then change the version selection to pre-1.16 and you'll see the fortresses did cluster in strips parallel to the z axis before, but they don't do that anymore. You'll see also that nether fortresses aren't generated as closely together as before; they are now less common.