Placing leaf blocks (IE in creative mode) into the world gives them the blockstate persistent=true. However, if the world is then closed and reopened, those leaves' blockstates change to persistent=false, as a result they then decay.
Steps to reproduce:
Start up Minecraft 18w22c and create a new world in creative mode.
Place down several leaf blocks of any kind, selected from the creative inventory.
Press F3 and note their blockstates.
Leave the world (IE "Save and quit to title")
Re-open the world, and look again at the leaves' blockstates with F3, and note as they decay.
Attached are two images of the same leaves, one taken before and one taken after reloading the world.
I literally just discovered this issue myself. I'm playing on my server which has an extremely high randomTickSpeed, meaning that the leaves disappear instantly once I either unload the chunks or exit the server. As such I had no idea what was actually going wrong until I visited this site and found this error post, so thank you for that!