It seems it has to do with the height map issue linked. I had this issue yesterday on 20w18a, from a world converted from 17a from 16a, and it got fixed on 18a after I optimized the world.
@bugsbugsbugs still working for you? I can only reproduce this on Creative, but on Survival it doesn't seem to work. Might be the random tick tricking me but I used a couple stacks of bonemeal already on a single tree and it doesn't grow.
Iñaki Agustín Guastalli
@Jason Mayer and all,
It seems it has to do with the height map issue linked. I had this issue yesterday on 20w18a, from a world converted from 17a from 16a, and it got fixed on 18a after I optimized the world.