Spigot, by default, disables several mechanisms around how chunks are
lit, if ever, which has forced them to always send chunks before vanilla
would consider them ready to send, causing for lots of issues around
lighting glitches.
Shamefully, the amount of work to relight chunks can be detremental
to some servers, meaning that forcibily disabling light updates can
cause major performance issues.
as such, we make a compromise; if this "feature" is disabled, we will
only send chunks which are actually ready to be sent, otherwise, we
will always send chunks.
Shane Freeder