I have experienced this problem on a skyblock world where I only have one platform which can spawn zombie pigmen. I can stand 2, 10, 100 blocks away for 5 minutes, 30 minutes, I left the game running for 3 hours standing over 100 blocks away in the nether and they never lost aggro or they would lose aggro and then as soon as I get anywhere within range the (I assume) newly spawned pigmen turn aggro again. This is not the way their aggro worked in any version previously and thus is not "working as intended."
I have experienced this problem on a skyblock world where I only have one platform which can spawn zombie pigmen. I can stand 2, 10, 100 blocks away for 5 minutes, 30 minutes, I left the game running for 3 hours standing over 100 blocks away in the nether and they never lost aggro or they would lose aggro and then as soon as I get anywhere within range the (I assume) newly spawned pigmen turn aggro again. This is not the way their aggro worked in any version previously and thus is not "working as intended."