Light level is set to level 15 in the night worlds, but it seems that grass does not spread.
I'm attempting to do a world where lore is based on eternal night, but my testers report that grass is not spreading even after a lengthy amount of time has passed.

It appears that this also applies to crops and trees. They will grow with torches next to them, but this is largely impractical.

All of the world provider settings are from vanilla Minecraft, OTG just lets you enable/disable them. Setting 'night world' to true enables world provider settings from the vanilla End dimension which has the same limitations you mentioned, this is not a bug.