Description:

Mojang team, here I'm talking about something that probably shouldn't happen. On 20w20b, zombies can spawn on Nether Fortresses and Bastions generated before 20w20b/20w20a. This probably is caused because the Nether generation changed many times.

The seed that I will show is: 8825242716498603998. This world was created on 20w17a/20w18a.
At world spawn, I created a portal, next I appeared on a Crimsom forest & Nether wastes, where I found a Fortress. I was redesigning the Fortress, replacing some Bricks, adding some other Blocks, and addes some spawners. But on 20w20b, I noticed that zombies can spawn on the Fortress. They appear next to some spawners. Also, when I was redesigning a Bastion, there also generated zombies.

How to reproduce:

1) Use seed 8825242716498603998, on 20w17a/20w18a (for the example).
2) At spawn point, create a Nether portal. There should be a Crimson Forest with a Fortress.
3) Modify the structure.
4) For bastions, just go to coordinates shown on images.
5) Finally, charge the world on 20w20b-a/20w19a.

On 20w19a/20w20a-b, the biome is a Soul Sand Valley, and there is no Fortress or Bastion. For that reason, I think that this is a problem caused by the changes on 20w19a, or before that version.

PD: I use spanish language, for that reason there is a blue part on spanish. We love Minecraft too. Thanks for hearing.