If you place a leaf block in a 1.12 world and upgrade to 1.13, that leaf block will start to decay. this applies to all leaves in the world
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Cannot confirm. Do you have a save of the world from 1.12.2? If so, please attach it here or provide a download link to the world.
I think my issue is related so I'll post what I happening. Mostly large/tall oak trees but other trees too, leaves decay apon generation. World generated trees are affected, no fire or lava anywhere near. Trees planted and grown are also affected. They leaves do drop items (saplings/apples) per normal. I didn't explore far enough to figure if other tree types are affected, but oak and birch I personally watched decay after I spawned in newly generated world (1.13pre3) I don't believe the world was generated in any previous version. I can post images and if you want world save
Looks like MC-130521, but there is 18w20 involved