Likely from the fix to MC-195461.
Despite cullface=up being clearly defined for these elements in both of these blocks' model json files, it is not executed, resulting in the faces being rendered at all times even when completely obscured and not visible.
No idea how this happened - almost all other blocks with upwards cullface (e.g. fences, full solid cubes) correctly render that face when an upper slab is above them, but cull it when that face is fully obscured.
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Can confirm in 20w51a.
Can confirm in 21w03a.
Can confirm in 21w05b.
Can confirm in 21w06a.
Can confirm in 21w08b. This did not happen in 1.16.5.

Because this is caused by the fix of MC-195461 in 20w46a
Can confirm in 21w10a. Video attached.

No, it does not.
Can confirm in 21w11a.
Can confirm in 21w13a.
Can confirm in 21w14a.
Can confirm in 21w17a.