If you stand at low y levels in void worlds in multiplayer, you will notice a blackness around the bottom of the sky as though you are in singleplayer.

Steps to reproduce

1. Create a superflat world with the "The Void" preset.
2. Stand on the generated platform. 
3. Do it again but this time in multiplayer.
4. Observe the differences

In singleplayer, the sky looks like this:

Whereas in multiplayer, it looks like this:

Screenshots were taken in 1.16.4 although the problem is also present in 1.16.4.