When I was destroying a village in creative mode, I came across this bug many times, when I removed a block holding a torch, sometimes the light would linger, even without other nearby torches. This mainly happened when I destroyed the torch's source block.
I know one of the houses still had a remaining torch, but 1 torch shouldn't create that much light, proving that such a bug exists.
Did you recently return from the Nether or End dimension (MCPE-145828)?