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I think this bug is actually easy to fix, or at least mostly mitigate. It can be fixed for old chunks and new chunks.

-When a chunk is loaded, load also the 4 adjacent chunks, and scan the borders for cut off trees.

-If one exists, regrow the tree, but only allow the regrown tree to replace air blocks

-Ignore chunks where the player has made modifications

-Maybe have some minimum size of the cut off tree that the algorithm tries to regrow (so a random leaf block or two, wouldn’t trigger a regrow)

This has the risk of breaking some custom trees, and builds based on leaves, but is probably better than immersion breaking broken worlds for everyone. To be safe, this can even be a custom setting that can be turned on/off by the player.

No, what Loic is saying is that, if when the game updates to a new version, minecraft were to mark all chunks as generated in the new version before loading the game, this problem may go away. That would be a pretty trivial fix.

This is a really horrible bug that really needs to be fixed. And it should be pretty trivial to fix based off of what Loic discovered.

I want to second prioritizing this issue. It breaks immersion, and after the Spring to Life release, seems to have gotten exponentially worse because that release seems to have changed tree generation. The issue shows up now on every chunk border between chunks generated before 1.21.70 versus those generated after.