Grass blocks (in the default texturepack and in other texturepacks - i checked!) on the side of the screen and a bit away from from the player seem to have a small black overlay, seemingly a texture glitch. This can be seen on the attached picture.
This also affects other blocks like leaves.
This doesn't happen if mipmapping is disabled, and higher levels of mipmapping make the issue more severe.
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What is "mipmap levels" set to?
Also, please attach a screenshot of this occurring while the F3 debug screen is enabled.
This issue is being temporarily closed as Awaiting Response. Once the requested information has been delivered, the report will be reopened automatically.
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Was able to recreate this issue. Only happens with mipmapping enabled.
Unsure if this is a new issue or not.

It's not new as can be seen in MC-114265, but it's extremely visible and disturbing as of 21w10a.

Despite the description, this is also on fabulous! Didn't test the other modes, but I think the various modes are irrelevant to this.

Threw in two screenshots, list of blocks affected include:
Grass Block
Iron Bars
Glass Pane
Tripwire Hook
Chain
Oak Leaves
Spruce Leaves
Birch Leaves
Jungle Leaves
Acacia Leaves
Dark Oak Leaves (which I forgot to include in the screenshot)
Azalea Leaves
Flowering Azalea Leaves

This and missing underwater fog are probably the biggest bugs I've seen with the new rendering engine.

I had the same issue when viewing the blocks almost parallel to the players POV
[media]Video attached.

Noticed this myself while creative building, on the side of dirt/grass blocks. (Added screenshots of it.)
Fabulous
Mipmap Level: 4
Smooth Lighting: Max

Felix has already mentioned that there is a fix in the works (which is a simple value tweak in a shader), posting more screenshots at this point won't make much of a difference.Β

Unfortunately this still seems to be an issue for me in 1.17 and above. It's not nearly as bad, but at a longer distances this darkening issue still occurs at many angles. Though it may be hard to see in screenshots, to me it's still quite noticeable in actual gameplay. Mipmap levels do not seem to have as much of a difference.
Β Sorry for all the edits, the images wouldn't upload properly for me. I've also attached grass 2 and 3 in the images above for further examples. This may be applying to leaves as well but I can't tell.
[media]
Actually my mistake, it seems lower mipmap levels DO indeed make it go away just like the original bug here. It's quite noticeable at a distance in swamps, and I can't see it occurring here. If it is, perhaps it's just too garbled to see.
This is still 1.17 by the way.Β
This issue only occurred in a singular snapshot. It's basically a worse version of MC-114265. Please go to that bug report for information about the bug that's still left in the game.

Mipmap Levels from: 1-4, are where the bug occurs.
Mipmap levels 0 fix the issue.
Β
Issue still occurs on latest 1.19.3 for some reason...