The bug
In your hand you can see that grass blocks are much brighter then when they are on the floor, in your inventory, or placed down.
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[media]
Can confirm in 21w06a.
Can confirm in 1.16.5 and 21w08b.

I am unable to reproduce this issue in 1.17.1.

This has indeed been fixed at some point. Was able to reproduce in 21w05b but can no longer in 1.17.1.