Leaves of trees generated in a very old save (presumeably Alpha) don't decay and don't drop saplings or apples when punched. Leaves generated more recently do decay and do drop items.
What I expected to happen was...:
Leaves decay when the wood is removed and drop items when punched
What actually happened was...:
Leaves float in the air long after the wood is gone and when punched they yield no items.
Comments 2
This is due to how data values were used in the past being different to how they are used now. You may also see that the old trees are made from various different leaf types.
Initially the data values (between 0 and 15) had something to do with how far they were from a wood block, for optimization of the leaf decay algorithm.
Now, however, it starts with a base value of 0, 1, 2 or 3 depending on leaf type. It then adds 4 to that if it was placed by a player and shouldn't decay. It then adds 8 to that value if it's currently decaying. The addition of 4 will override this.
Obviously, this new functionality can't tell the difference between old and new leaves, it only knows that they have a data value between 0 and 15. So, the famous computing phrase, "garbage in, garbage out" applies here.
Tl;dr- The data is too old, look at the top half of http://3.bp.blogspot.com/-iINAZA0i-ps/UwaTF0o3-KI/AAAAAAAAIbY/dKBoDi02G4I/s1600/garbage_paradigm.gif
This is not the fault of the current Minecraft version.
Something went wrong somewhere in the process updateing the save ganes.