The Nether generation in the lastest snapshot seems to be incomplete. Actually it looks like it keeps the 1.16 biomes and blocks but is generated with the terrain of former versions.

For instance, the warped and crimson forests don't contain any trees. Same for the basalt deltas, they have none of their usual structures.

Bastion remnants are still there though.

 How to reproduce it : nothing special, it seems to happen on every map (tested on singleplayer & multiplayer)