Placing leaves with /fill where there were already leaves will cause them to decay

I was building a wall of leaves with the /fill command, and the I switched to jungle leaves and they started decaying.

What I expected to happen was...:
They should not have decayed.

What actually happened was...:
They started decaying.

Steps to reproduce:
1. Place some leaves with the /fill command.
2. Notice the leaves do not decay.
3. Repeat the command (you can change the leaves type).
4. Notice they do decay.