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