When a block is turned into a chiseled block, the blok immediately becomes darker and almost emits negative light.
All chiseled blocks that have a relatively small amount of bits removed are very dark.
There seems to be a relation in how many bits there are in a block and how dark they are.

This happens in a clean Minecraft instance without any other mods

Here you see the dark chiseled blocks:

Here you see a similar structure in Minecraft 1.12: (Notice that the gaps aren't dark)

• MC Version: 1.16.5
• C&B Version: 1.0.12
• Do You have Optifine: No

Yeah I am running a test version which cleans up the configs internally.
But nothing more.

So I have no clue what the cause is.....
Let me do some testing, but I can't promise anything and it might in the end be related to something that is not under my control.

Interestingly my forge config file is different

Are we running a different version of forge or something else

I remodelled this and I was not able to reproduce this.
Can you do two things for me?

Make a screenshot of your graphics/videos settings.
And check if you have the forge pipeline enabled.

I looked up the forge light pipeline in the config file and found:
experimentalForgeLightPipelineEnabled = false
forgeLightPipelineEnabled = true

So I enabled experimentalForgeLightPipelineEnabled

Now when I load back into my test world, the lighting seems to be fixed. But when I break and replace any blocks it becomes dark again. However less dark then before editing the config file

Just after loading in:

When I replace all blocks: (Note: I use middle click for copying chiseled blocks)

Also here is a screenshot of my settings: (Yes i run my brightness in moody, but when I turn it to bright it is still to dark)

here is a screenshot with the config disabled again with brightness turned to bright

Can you see if you can reproduce this with chisels and bits only. And then send me your world.

Cause I don't get this at all and I think it might be related to model data somehow

Since the Minecraft instance I was using only has Chisels and Bits I just sent you the word where I have been testing in
C&B Test.zip
.

Original freshly loaded into the world:

New one rebuild just as you described:

As you can see I don't have any issues like this.
My test graphics settings:

And these are my settings for the forge client:

As such I am not sure what the cause is.

Would it help if i sent over my whole test instance?

I use MultMC by the way

Also have you tried setting the brightness to moody?

And here is a new clean instance where I reproduced the issue:
C&B 2.zip
I used the latest version of forge that i could select in the MultiMC options.

I did my best to fix this in the newest release.
There is not much more that I can do since I can not reproduce this on my end.