The world will take a very long time to generate and will get stuck on "Building Terrain" after about 10-15 minutes.

How to reproduce:

1. Grab your switch and open up Minecraft
2. Generate a world with GameTest, additional modding capabilities, and custom biomes turned ON in the "Experiments" tab. Make sure the world type is set to "Flat"
3. Attempt to generate the world.

Note: Attempting to close the game while the bug is triggered will cause Minecraft to crash.

Additional notes:

I have a hunch that it might be the "Custom Biomes" feature, so I'll be sure to test it with only that, and my results will either be in an edit or in the comments if editing isn't available.

Test results for Custom Biomes: No change from standard.

Test results for GameTest: No change from standard.

Test Results for Additional Modding Capabilities: No change from standard.

Results: Must be a combination of the three that's causing it.