How to recreate the bug:

First create two or more custom biomes with the custom worldgen datapack feature.

Next, import the datapack into the create new world menu.

Then, select single biome in the world type settings.

Generate the world. The world will look and act like it generated properly for the custom biome.

Finally, log out of the world and log back into it. The biome will be set to a different biome in the list of custom biomes made in the datapack, and not the correct custom biome. I have found that it occurs with certain custom biomes, but not others.

I have also experienced the same glitch where a custom dimension will mix up custom biomes as well. However, I have only experienced it once.

The pictures included illustrate the problem. The first picture is what the custom biome should look like. The second picture is after I have logged out of the world and returned to it. It now says that the world biome is another one of the biomes I have created, and acts like it as well.