The Bug
Birch forest biomes generate in areas with a high humidity level, which forces them to generate adjacent to Jungle and Dark Forest biomes very consistently.
Reproduce
1.) Open a new creative world with seed 455660275549672724
Fly to 300 / 77 / 950
Open debug menu to view high humidity value
2.) Open a new creative world with seed 1534726942432747233
Fly to -355 / 117 / 905
Open debug menu to view high humidity value
Expected Result
Birch forests generate in areas with lower humidity and rarely border Jungles and Dark Forests
I believe this is working as intended. Birch forests commonly generated next to dark oak forests and jungles in the previous versions (1.7.2 to 1.17.1). Example: seed 1, (1) birch forest borders jungle (at jungle edge) near 1368 -432. (2) Birch forest borders dark oak forest near 104 -1244.