Zombie Pigman are not naturally spawning in nether portals, they can move between them but on the wiki and in other versions on Minecraft, zombie Pigman (on easy mode) have a 1/2000 tick chance of spawning per each nether portal frame that is built (so if there is 10x10 lit nether portal frames, that means in easy mode there should be a 100/2000 tick chance that one Pigman will naturally spawn. I have made a creative world, built a 144 nether portal block frame, and waited 30 minutes with not a single Pigman spawning. Hopefully this can be fixed soon