The bug

When you use the /fill command to create a room, or use a structure block to load a structure, the lighting will not properly update inside of the structure.

To reproduce

1. Go into the sky
2. /fill ~ ~ ~ ~15 ~15 ~15 minecraft:white_concrete hollow
3. Enter the concrete cube
   → Occasionally the room will be partially bright inside. If you open the F3 screen you can see that the client and server light values do not match.