After cut many tree no branch seed to fall. All leaf disappear after time but no small branch found on the ground to plant others trees.
More info:
This is in survival mode and I got only one saplings after cut around 20 small Oak trees.
NOTE: If you don't cut the tree and directly destroy leaf, saplings was drop.
If you did not already, please remember to perform a search before creating a ticket to see if your bug hasn't already been reported.
This issue was reported previously as MCPE-8232; comment your experience there!