Otherwise azelea trees won't have azelea leaf piles on worldgen and there wouldn't be leaf piles above trees (for example villager houses).

The current leaf pile system is working as intended.

Again, this is still working as intended. You can't just modify the base tree feature like that even if I wanted to. The leaf piles form below trees over time, there are also leaf piles that generate on forest floors to add some variety.

These trees don't have leaf piles under them on worldgen.

These leaves generated on a place where they should not have generated.

Wouldn't it be better if these leaf piles where generated together with trees instead of being a seperate feature?