So far I've just encountered this bug once. Poise forest mobs will spawn and the skybox and soundtrack changes but nothing else. The biome is scattered around the area specifically associated with the gravel beaches. stepping into the nearby grass changes the biome back to birch forest.

EDIT: After investigating further this seems to not be an issue with the mod, recreating the world in Single Player with the same seed changes de biome to Gravel beach. Must be an issue with our world generation since some areas have the wrong biome, probably after updating the forge version. Still I find it weird why would an end biome end up in the overworld.