Steps to reproduce:

1. Create a new world, load a pre-existing one (1.15.2 worlds have this issue too) or log onto a server.
2. 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.