This lighting bug might be related to a couple of others I've seen that are open, but those talk about either skylight issues, or about lighting sources going "out" (like torches). The cause of this bug could be related to the causes of the others, but I'll let a developer figure that out.
What: in a pitch black hole underground, put a single log into a furnace and smelted with a single plank to create charcoal. Room lit up as furnace operated. After furnace finished and went out, noticed that there was still light in the room. Looked up F3 and server block light is 0, client light/block light is 9.
Predictably, breaking the furnace updates the light. Reproducable (see video).
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I have been messing around with this for a while and I can't reproduce it. Started to wonder if it was related to chunk lag as has been described in another bug report, so I traveled several thousand blocks but still nothing. Only one update I can add at this time; if you watch the video, you can see at one point only half of the room is lit up by the furnace while the other half is dark. Double-checking coordinates I can confirm that issue happened at the edge of a chunk boundary; the lit up side is in one chunk and the exact spot the darkness starts was in another chunk.
I'll keep working on a reproduction path. Maybe someone else will have luck corroborating this and narrowing down the conditions that caused it.
Please check if that's still an issue for you in the latest 1.14.4 stable release or the latest 1.15 development snapshot.
https://youtu.be/YHmJ4_fAHyk