Leaves are not updated to the new block arguments when updating

Please attach the crash report ([minecraft|http://hopper.minecraft.net/help/finding-minecraft-data-folder]/crash-reports/crash-<DATE>-client.txt), otherwise we can not diagnose this issue.
For technical support please use the Mojang Support Center.

-- I am a bot. This action was performed automagically!

decayable=true -> will not stay -> persistance=false
This is correct.

But leaves with either check_decay or decayable set to false will not decay in past updates. Most of my map fell apart when I updated because of this change

check_decay completely got replaced with the distance block state; you should’ve just use decayable if you want persistent leaves.

Actually, no matter what arguments the leaves had in 18w21b, they will automatically be set to persistent=false. So even if you placed the block by hand, it'll disappear. Is this intended behaviour?

Vanilla data pack disabled? See https://bugs.mojang.com/browse/MC-130419?focusedCommentId=456446&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-456446

Indeed I have. I'll enable it again and try to update from a backup.

In that case, it's a duplicate of MC-123029.