When playing on a server, I noticed low spawning rates on a dark room based mob farm that worked fine in single player. When I joined the server while nobody else was on, mobs were spawning as ordinary, but as soon as another player joined, and I mean in the literal second, mob spawning nearly stopped happening. It would no longer do anything.

How to reproduce: Create or play on a dedicated server. Build a dark room based mob farm high up in the sky, above the ocean or any other place where it would make sense. Look at mob spawning rates. Have another player join the server. Look at mob spawning rates again.