I created an automatic chicken farm that would reproduce endless amount of chickens to help me gather wither roses on multiplayer. The chicken farm ended up creating 9.5 stacks of wither roses the first time around. I vaguely noticed that there weren't mobs spawning but I was working on a separate project in a safe area so didn't think anything of it.

The second time, the farm produced about 7 stacks of wither roses. During this time, I was in the nether and obviously noticed that there were no mobs spawning. I sent a message to one of the other players and he noticed the same thing- no mobs no where. 

The third player was sitting at the chicken farm in order to continue growing the chickens. Upon moving the the third player, the other players were able to start seeing mobs again. 

Not sure if this was an intended issue or not as I couldn't really find anywhere that says mobs stop spawning across the entire server. 

Replicate- set up an a chicken farm capable of spawning large amounts of chickens automatically and stand by it. Have a second person go anywhere else in the world. The second person will not get spawns either.

Fix- The first person walks away from the farm.