In the example provided, since it cannot be placed on the side of a sapling, it would be expected that it would instead attach to one of the solid blocks nearby that can support it, but this does not happen despite being the case for almost every other block.
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discovered while testing
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[media]
Can confirm in 20w51a.
Can confirm in 21w05b.
Can confirm in 21w06a.

Can confirm in 21w42a.