On some chunkboarders, the leaves don't generate. I've seen it happen where the tree is generated but half of the leaves aren't but also where only half of the leaves has generated but the tree haven't. This error doesn't continue on but stops at some point.

I've tried to recreate it, but I couldn't make it happen again. This makes me think it has something to do with some chunk loading error. At first I thought my computer just were crap and had difficulty loading the world, but then I also saw it in xisumavoids snapshotvideo of 21w15a.

https://www.youtube.com/watch?v=x8dP2-vI1lc

you can see some trees in the background with some leaves missing at 9:01. And he point it out himself at 9:08.