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.

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