The lighting is updated poorly and this results in spots with no light. A way of fixing it, is by updating the lighting which means you gotta to break the blocks that render dark. A proper fix is needed!
This bug also happens in old versions of Minecraft, but I thought it has been fixed by now.
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it happens when using /fill command. Sometimes it will be light coming out of nowhere or just dark spots

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We're tracking this issue in MC-172980, so this ticket is being resolved and linked as a duplicate.
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Update: after some testing, this bug also occurs when generating new worlds in creative
Did this happen when generating the world? Or did you execute any command?