Mobs enter the creaker forest at the time when it is necessary to get rid of the creaker and this deprives the opportunity to feel fear. It would be possible for mobs to avoid the creaker forest using the avoidance fear system, but this would burden the world of Minecraft. I suggest you reuse the mechanics of calculating the path through hot or dangerous blocks. That is, a block of gray forest grass was perceived by mobs as a lava block that cannot be stepped on and they will not step on it.