I reported this before for 1.16.2 pre-release 2 and it got fixed in MC-196743, but when I tried to make a new biome and a single biome world out of it in 1.16.5 (after I hadn't done it in a while) I got "Received invalid biome id: -1" in the log over and over and the game stopped responding, just like before. I also tested it in 21w17a to make sure it hadn't been fixed by then and I got the same error.

I couldn't go back and test the pack in 1.16.e pre-release 2 because file locations and stuff for custom biomes/dimensions has changed since then, but since my biome works in my custom dimension but not in single biome worlds I'm pretty sure it's not an error in my data pack but an error in the way the game is loading biomes. In MC-196743 I mentioned modified overworld biomes not working, which may still be the case but I haven't tested it.

I'll include the data pack I'm using.