Forest biomes seem to generate on hilly areas when both the T (temperature) and H (humidity) noise values are 3. On non-hilly terrain at these T and H values jungles and sparse jungles generate, while forests only generate when humidity is 2, but in hilly areas where humidity is 3, they still generate and replace jungles, which not only looks a little out of place but also creates microbiome issues, makes the forest biome too common and restricts the jungle and sparse jungle biomes from generating in hilly areas.
[media][media][media][media]By looking at these images, it's possible to see that the T and H values in the biome builder line in the debug screen don't change.
Seed: 1912783273887494969
The coordinates are in the images
I've also noticed that it isn't even consistent with jungles at a different humidity. Forests will replace jungles in an unsightly manner when the humidity level is
[media]3, but not when it's4. The seed-2791560360009905237and the following command will place you at the border of a jungle and a forest where there should be jungle: