I encountered this bug by joining a multiplayer server, other people on it seems to get the same result. It is a minecraft vanilla 1.14 server and the bug seems to not affect every chunks.
Disconnecting and reconnected or rebooting the game does not seem to help.
Replacing the light source restore it to normal.

Edit: breaking a block next to the light source seems to work.

Edit2: It allows mobs to spawn.