This issue occurred a few times, but reloading the world or the chunks fixed this. This is a really minor bug and I don't know if you want to fix this.
Environment
Win 10, Java 1.8._51 64bit, Minecraft 21w08b/vanilla/snapshot
MC-217353
Two areas with light level 0 at daytime
Resolved
Duplicate
0
Duplicate of MC-214793