If you count the amount of pixels from top to bottom of the potted cactus, they total to 17, even though blocks should only be 16 pixels tall.
A fix for this can be found attached to MC-194192
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In 1.16.2 Release Candidate 1.
Can confirm in 20w51a.
Can confirm in 21w05b.
Can confirm in 21w06a.

In 21w08a

In 21w11a
Can confirm in 1.17.
In 20w29a