I don't know how exactly how this appered, but it properly appered in '18w06a' where the world generation code was change, any ways tree generate with no leaves on one side or the other side.
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Could not confirm for MC 1.13-pre2.
I have tried to reproduce the bug and I did not find the "half tree". However, I've already seen it.
This is a generation error, as you said, but it does not always appear.
Thank you for you report.
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I’ve seen this on a multiplayer server of 1.13-pre1 as well. The leaves are missing from a neighboring chunk when the logs spawn directly on the edge of a chunk. Never thought to check and see how frequently it happens or if it’s only specific types of trees or specific chunk edges.