I used /fill command to fill an area in a brand new world. It looked fine underneath, however the debug screen read light level 0 and mobs were spawning.
MC-152863
/fill command leaves lighting errors
Resolved
Duplicate
0
Oli Ford
(Unassigned)
Unconfirmed
Commands
Minecraft 1.14.2 Pre-Release 4
Retrieved
Pretty sure this is a known artefact of the lighting system, should be fixable with a relog