I came across this on a world file that was updated to 21w15a from 1.16.5, when exploring newly generated chunks. It seems like a lighting glitch that results in random areas having light level 0. I found that updating blocks (i.e. breaking dirt, etc.) bordering these areas causes portions of it to correct itself. I also found that restarting the game seems to fix this as well. This glitch doesn't seem to occur in chunks loaded before the update to 21w15a.
Environment
Windows 10, Java Edition (Snapshot 21w15a)
Duplicate of MC-214793