Currently I've found 1 tree's leaves that don't seem to decay, I'm not sure if you can fix it or not, but thought I'll just point it out. The other trees' leaves do decay, but I haven't tested them all yet.

Seeing the same with Dark Oak Leaves; "minecraft:leaves2:13" according to TellMe. I am using a 'fast decay' mod.

We're seeing this as well, but so far only for spruce leaves. I live in an "Uplands" biome and naturally-generated leaves never decay, even without fast leaf decay. I have to burn them down.