I have this issue too. I have a big platform above the nether, right against max height, so no close spawnable areas. When I arrive, pigmen spawn, but they will not continue spawning once those initial pigmen are dead (despite the platform being 32x32). Entity count is near 0. I'm the only person on the Realm. I downloaded a copy of the world, and pigmen spawn fine in singleplayer. I believe it's a realms render distance thing? I have no solutions, and am wondering if I should file my own issue about it, since this one is closed (unresolved)?
Mick, I believe you're right and it's a server render distance problem and is persistent in 1.12 and 1.13. It's a shame, because I hoped Minecraft Realms' extra cost would allow it to be more functional and easy to use and have more support than a standard server-hosting site. Hopefully they fix this, or I too will be switching away from Minecraft Realms.