Mob farms do not work right on realms. i.e. my gold farm on my realm only spawns a few pigmen, and their agro doesn't last more than a few minutes at least. But on one of my single player worlds, it works perfectly. Other people have had the problem too, and were asking about it in the comments of the youtube tutorial I built it from.
Same thing with other farms, not very many mobs spawn, and the realm is on hard.
https://www.youtube.com/watch?v=l9oteFEj770&t=
^^Above mentioned video
Comments 3
Is this still an issue in the latest version of the game? Also, are you playing on Bedrock Edition or on Java Edition?
Cleaning up old tickets: This ticket had been set to 'Awaiting Response', but has not received a response from the reporter (~3 months+) so is being closed as Cannot Reproduce. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines which includes steps to reproduce the problem.
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I've also experienced this problem since about 1.12. It appears to be related to mob cap when more than one player is connected to the realm.