[MC-142134] Light sources spontaneously not working in some chunks

Moderator Notice

In order to fix your existing world, you can try following these steps. Apparently this is just a temporary workaround, so you might need to repeat this once the issue reoccurs again.

Singleplayer: Select world → Edit world → Optimize world → check "erase cache data" → Backup and load

Multiplayer: Start the server.jar with the following command:

java -jar server.jar --forceUpgrade --eraseCache

This only needs to be done once, after this you can start your world or the server the usual way again.

Minecraft Realms: Download the world from your realm, copy it into your Minecraft singleplayer worlds folder, follow the steps for singleplayer, and then upload the world back up to your realm.

Please read

If this is still happening to you, and if you have a backup of the world right before you can trigger the issue, that might help; I managed to get one and consistently reproduce it, and fixed what was causing it, but there might be more to it; if you can reproduce it by

Restoring the backup

Choosing edit -> optimize -> erase cached data (before loading it)

See the issue immediately after loading the world

Then it would be very helpful if you can share your world backup, along with what render distance you're using.

Thanks!

– [Mojang] Georgii Gavrichev in this comment

I found a stairway I'd previously made now pitch black.

I later found other areas (entire chunks, surface to bedrock) showing this behavior as well.

This is not just a visual glitch - the light sources I've used (torch, lantern, and jack-o-lantern) are all nonfunctional. As a result, monsters will spawn in them, and underground crops in these areas will break.

Forcing every single light source to update (or outright replacing them) is the only way I've found to get them to visually working again.