Trees often generate low enough in the ground that only the leaves are above ground level (or flush with it), with often a single wood unit generating below ground level.

Presumably the wood part of most trees should always generate when surrounded mostlly or entirely by air, and the leaves should presumably only (or mostly just) generate with an air gap of at least 1 block above dirt.

I think we started to see this generation weirdness with 1.18, but I'm not sure (may have been 1.19)