Possible bug I've noticed or maybe it's intentional.
Trees spawned into the world by the game, no matter if they are 300, 600, 900, or 1200 hp, if they are the correct type of tree, will end up giving essence of whichever type they are. (Granted you need the correct level of essence mastery.) If a player plants an oak, pine, or blue spruce seed, chopping down the tree while it's still growing (300, 600, or 900 hp), will result in no essence being harvested at all.
I personally have no issue with you saying it is intentionally done this way. I mean, if I've already taking the game time to make tree farms and I haven't hit day 14 yet, I can take the time to wait for trees to be fully grown.