When I was trying to build a tree house, I noticed leaves decaying even though they were connected to wood. I went onto creative mode and determined that the issue happens when wood is placed on the leaves then broken.
How to reproduce:
1. Plant a jungle tree (I haven't tested it with other types)
2. Place a log then destroy it.
3. Leaves will start to decay
1st image: tree before testing
2nd image: placed a wood plank then removed it, nothing happened
3rd image: placed a log then removed it, leaves started to decay
4th image: tree ~60 seconds after removal of the log