[MC-142228] Skylight shines through blocks

Mod Notice

If your world is still affected by this because you upgraded it from 1.14, please follow the following steps in order to fix your world:

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.

If the issue occurs again after you've done these steps, please leave a comment.

I was making giant enclosure in the sky for mob spawning so they would path into water and fall enough to almost die. However, I couldn't get it to work as for whatever reason, some blocks saved the light level of the sky during day, re-emitting it. So even at night, mobs don't spawn except in maybe a corner. The issue seems to multiply every time I relog, as I've made effort to fix the issue by replacing blocks, and when I have finally managed to fix it in a section, a relog caused it to come back. This happened with regular blocks, slabs and double half slabs.

As you can see in the screenshots, there was a corner where mobs could theoretically spawn for whatever reason. This was also during the night, so it's not just blocks letting the light through. Hopefully this will be fixed in a way so I don't have to replace so many blocks when a fix rolls around.