Minecraft:

• Minecraft: Java Edition 1.14
• Chunk Radius: 16
• Map Difficulty: Hard

Once a large group of Zombie Pigmen chases after a player, they will not turn passive.
 

To Recreate:

1. Start with a world in Hard Difficulty mode.
2. In creative, portal to the nether.
3. Create a clear escape path ~200 blocks away. (In my tests, their still exists space for pigman to see and spawn around this path; it is not enclosed.)
4. Switch to survival mode.
5. Near the portal, harm a zombie pigman and pillar up at-least 5 blocks.
6. Optionally/Alternatively: You can create a box around yourself so the zombie pigman can't see you. I don't believe this makes any difference.
7. Wait as many more zombie-pigmen huddle around your pillar.
8. After about 2 minutes from the initial attack, with a large group of pigmen below, run down your escape path and suicide (you can wait a minute if you wish, I don't think it matters).
9. Re-enter the Nether,

Actual Outcome: Zombie pigmen will still try to kill you.

Expected Outcome: The pigmen around the portal should be passive because (1) you waited over 40 seconds after the initial attack and (2) had retreated over 40 blocks away.
 

• I would guess that the hostile-timer is being reset as newly spawned pigmen arrive and "pickup" the hostile behavior.
• It could also be, although seemingly more unlikely, that the zombie pigman that were hostile before the players suicide, somehow re-hostile the pigman upon re-entering the portal (being in the "loaded" chunk radius even though they are 40+ blocks away).

Note: Back in the overworld, when I have zombie-pigmen aggro against me, I pillar-up and they soon become disinterested and walk away.

Note2: During testing, upon re-spawning, any zombie-pigmen ALREADY in the overworld will become aggro for no reason (they somehow pickup the aggressive flag from those in the Nether).