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.