When cutting down a tree its leaves won't fully decay unless the neighboring trees' logs are cut as well.
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A tree is nothing more than a collection of blocks, not a single one of those blocks contains a "parent" property. Leaves only check their distance from wood/logs and decay if too far away. This has been the case since addition, and remained the case during their block state rewrite in 1.13.
Absolutely intentional/Not a bug.