On Spigot, a lot of lighting glitches are happening in the form of completely dark blocks in some dense areas. This is particularly visible in jungle or caves. It happens on newly generated chunks and on already generated chunks.

I guess it's a Spigot issue because it happens A LOT less on CraftBukkit "vanilla", but happens on Paper Spigot too for instance.

I attached 2 screenshots of the exact same world running at the exact same time on both Spigot and CraftBukkit. Light glitches on these screenshots happen on already generated chunks.