The bug

When a block on fire is broken, the light of the destroyed fire is not updated.

How to reproduce

Put a tree on fire and wait for the destruction of the blocks and you will observe the not updating of the brightness level (also affects the menu F3).

Note

From PhiPro:

This is partially resolved in 18w44a (iirc already 18w43c).
Lighting is now correct clientside. However according to F3 it's still wrong serverside.
Relogging fixes the serverside part. However, at least for spawn chunks, teleporting away and returning won't fix the issue and the wrong lighting is then actually synced to the client.