I was doing nether advancement in 20w27a and 20w28a is uploaded. So I got a new snapshot version and upgraded my world to new snapshot 20w28a. And I went to the nether and there was warped_forest, the place where is my nether portal is. A few seconds later, in my house in nether, some white particles began to fall down. I went to that area and it said Advancement Achieved "Hot Tourist Destination"!!! Before, I had traveled Crimson and Warped_forest, Soul Sand Valley, and Nether Wastes but no Basalt Deltas. I was confused and did /gamemode spectator but I couldn't find any Basalt Deltas structure.

*********Minecraft System also says Basalt Deltas*************