Loading the 1.17 generated world into the 1.18 beta is causing leaves to generate in old chunks. I had a hypothesis that the world was placing trees/leaves from the new generation into the old. I loaded up a new generation of the same seed and found that there is a spruce forest that generates in the same location as the spruce leaves are appearing in the original world. Upon loading into the old world again there are now even more leaves and even a half tree that has spawned into the old world. 

World seed is 23