In my Survival Realms world, 4 wolves, a villager, and a portal-spawned Pigman entered the Nether portal. When I tried to push them back through, however, it caused all entities to lag, and they never entered. I did wait out the 15-second cooldown. I tested it in a private Creative world and it worked perfectly except when I didn't first load the Nether. Perhaps the Overworld doesn't load properly in Realms?