The bug
Whenever you create a flat world, everything is suddenly dark until you fly up.
Linked issues
is cloned by 1
is duplicated by 20
15 more links
relates to 2
Attachments
Comments 14
See also MC-118103, although it didn't affect superflat worlds previously afaik.
seems to be fixed in 1.16 pre-release 1 (try to generate superflat void preset and get to at least y0)
Possibly a reappearance of MC-34889