Final Conclusion: I've been playing on my realm ever since I started this issue thread and I have every detail on the issue. When the realm map converted to 1.13, everything was normal until a few days in when my game crashed and the realm crashed. I logged back in after starting up Minecraft again and the entire world, except for an entire chunk line where I was before the crash, had been reset. I had to go to the world backups and load a previous save to undo what had happened because everyone who had previously played, had lost everything. This might be another bug in itself, but it brings me to my main point. I've spent a lot of time in that, and the surrounding area, meaning a lot of the map had been loaded before 1.13. In these areas, the mob spawning is bugged. When not moving (while being inside a 1.12 loaded chunk), if I where to write /kill @e[type*every hostile mob*] I would be seeing a massive decrease in spawn rates, as hostile mobs basically would not spawn anymore. As is for the nether dimension. When using the farm as described below, I would be standing afk in the middle of the farm, all the pigmen would run towards me and get killed, but then none would spawn except a few. Once they've died off, none others would spawn eventually. Except if a player is moving. Either I have to move around the area for mobs to spawn, or another player has to. I have tried the exact same farms, re-built inside newly loaded 1.13 chunks and they're working as they should. I don't know what is causing this issue, but maybe if the same option that was added to single player, where to you optimize the world, were added to realms it would resolve the issue. Really hope you guys at Mojang can somehow figure this out and maybe fix it, as a lot of time has been spent on this map and restarting is NOT an option. Our goal when starting the realm a couple months back was to never start a new world anyways. If a video would be helpful to understand the issue, I'd be more than willing to upload a clip to youtube explaining and showing everything in detail.

ORIGINAL POST: Mobs spawn way less in the nether. I have a pigman farm, designed by ilmango (if you need video for reference), and in the previous version it would spawn at least 50 pigmen every time the first batch started dying. It was very good and a lot of pigmen spawned, that is my point. In 1.13, maybe 3-4 spawn every minute. And if another player joins, pigmen wont respawn at all. The farm uses iron golems to draw magma cubes off the farm, but they don't seem to spawn at all. There are also pigmen spread around outside the farm, but both those and the iron golems are named and should not count towards the mob cap. This farm is over 200 blocks in the air, but I also find that nothing spawns in the nether in general.