When comparing identical seeds between versions it would appear that the general features (caves, plateaus, lakes) of the nether dimension are very similar, and that for nether fortresses that do spawn, they spawn in exactly the same way with exactly the same shape in exactly the same spot as in 1.15.2.

Yet it would appear that many fortresses are failing to generate.

When testing in seed -8263492127909406043, the fortress at 587 70 984 spawns in the same place with the same rooms and the same hallways leading out. Essentially an identical fortress. Yet the fortress that is found at 903 68 630 in 1.15.2 does not spawn in 20w06a. In fact that fortress has 2 neighbours in the same lava lake to its north and south which also fail to spawn in the snapshot.

Before this point, those fortresses properly generate identically in all versions from at least 1.11.2 to 1.15.2

Thus potentially looking at a massive reduction in the number of fortresses that generate.

How to reproduce

1. Create a new world with the following seed

   1894983001

2. Teleport to the nether using this command:

   /execute in minecraft:the_nether run tp @s 3314.70 105.35 -4676.21 273.45 82.50 

   There is no fortress here, unlike in 1.15.2