I've noticed that going below Y61 creates a black horizon/sky.
This hasn't happened to me on singleplayer, but on multiplayer it seems to happen for some odd reason. The server is hosted by a external host, maybe it is something on their end. But they didn't have this on 1.15.2 iirc, so I'm uncertain if this is a game error, outdated hardware for the server or a mixture of both.