Similar to a Tic-Tacs issue, but not the same. The following is copy-pasted from my respective issue on C2ME, with some modifications:

Chunks are completely dark, on all y levels, when paired with C2ME (cannot reproduce with Phosphor 0.7.0 and below). Happens in patches, which can be different sizes (e.g. some are 3-4 chunks, some are 7-8 chunks), and patches are not very common, (these are approximations, and you should 100% reproduce more scientifically and look into the code rather than going off my measurements). Bug is similar to another issue, but is less severe, e.g. both skylight and blocklight are affected and occurs in chunks. You may need to fly around for a few minutes to reproduce, or sometimes you can reproduce quite quickly.

Hardware:

• i3 6100U (also reproduced with i7 7500U)
• 8GB 2133MHz RAM
• Intel HD Graphics 520 (also reproduced with Intel HD Graphics 620)

Software:

• LMDE (Linux Mint Debian Edition) 4 (also reproduced with Windows 10 2004)
• MultiMC 5; 0.6.11
• Minecraft 1.16.5
• Fabric 0.11.1
• OpenJDK 11
• C2ME 0.1.0-rc4
• Phosphor 0.7.1

Screenshot:

That mod wildly throws threads at everything in the name of performance: I would really not advise using it

I am aware that is it incomplete/WIP, unstable and not recommended to be used. I was simply doing some testing for C2ME and noticed that Phosphor caused this issue. I just wanted to make sure that this bug can be seen by multiple sets of eyes from multiple angles, so that if/when C2ME becomes stable/usable/recommended, this bug doesn't persist.

That mod wildly throws threads at everything in the name of performance: I would really not advise using it

Closing this as it is most likely a pure C2ME issue, see RelativityMC/C2ME-fabric#7 (comment).