Opened the game: https://paste.ubuntu.com/p/hr2G3TnJBW/
A couple problems with the next few steps:
1. I had to force-quit the game to capture the logs as it was getting spammed with thousands of
Received invalid biome id: -1
warnings
2. Although I can select all the text in the logger, I can't cmd-C copy or right-click copy all the selected text; it only copies the text that is visible on screen.
I've tried my best to get stuff that might be helpful despite these two things:
Creating the world: https://paste.ubuntu.com/p/mGQ7ZFMys5/
Creating the world still + just entered world: https://paste.ubuntu.com/p/HDyyddkVDt/
The rest of those logs are 18000 occurrences of that warning.
Opening the world a second time: https://paste.ubuntu.com/p/WvgCQ45rqj/
Opening the world a second time part 2: https://paste.ubuntu.com/p/4byjwrwB62/
Creating the same world in 20w30a for comparison: https://paste.ubuntu.com/p/sMnxNCvJY7/
To confirm that it's not just the specific custom biome, I went and created a "Swamp with no creepers" biome and confirmed that the same issue happens with that one selected instead.