By using falling sand in a specific contraption, a interesting visual glitch occurs.
Also, when falling sand goes through a block, it produces z-fighting.
Please watch the video and look at the pictures attached, it should explain everything.
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Can confirm in 21w06a.

Cannot reproduce in 1.17.1.
Similar to @unknown, I cannot reproduce this issue in 1.21:
[media]
Is this still an issue as of 1.21? If so, then please add it to the Affects Version/s field.
This issue is being temporarily resolved as Awaiting Response. Once the requested information has been delivered, the report will be reopened automatically.
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Cannot reproduce in 1.21, either. This post should be able to be remarked as resolved
Can confirm in 20w46a.