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 But they didn't have this on 1.15.2 iirc, so I'm uncertain if this is a game error, outdated hardware for the server or a mixture of both.
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-118103, which is a report that has not been resolved yet.
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