i was using fertile soil in my automated tree farm which uses origin saplings i noticed when the sapling turns into a tree it turns the fertile soil into regular dirt which it shouldnt be doing ive used it before in several packs and it kept the soil

This is a known issue from Biomes O' Plenty, it only happens with certain trees and there isn't much we can do to fix it, unfortunately. It only seems to happen with oak-like trees (i.e. cherry, jacaranda, origin, rainbow birch) so best to avoid using those in your tree farms.