Affects Version/s: Minecraft 1.14
Fix Version/s: None
Environment:Windows 10 Pro 64-bit
Intel i7-8550U @ 1.80GHz
Radeon RX 550/Intel HD
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.