Steps to reproduce:
Create a new world, load a pre-existing one (1.15.2 worlds have this issue too) or log onto a server.
Use
/fill ... white_concrete hollow
to create a box with dimensions of 7 or more blocks in any place in the world previously exposed to light.
Expected behaviour:
The box is dark inside.
Result:
The box still has skylight level 15 inside of it.
Fix:
Relog into the world/onto the server or rebuild the box.
History:
This issue was introduced in 20w12a, but only affected chunk borders (second screenshot). It was fixed in 20w15a, until being reintroduced in 1.16-pre1 (first screenshot).
Notes:
This doesn't work with all kinds of blocks. It does work with glowstone, dirt and concrete, but not with grass blocks.
If the box is not a cube, it will be lit on one side.
Updating lighting inside the box does not fix the issue.
World type does not matter.
Game rules don't affect this.
Sometimes randomly doesn't work.
Linked issues
Attachments
Comments

Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-186337.
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