In Nether-realm:

Sometimes, when striking a pig-man, the anger cool-down does not come into effect.

waiting 5 mins without harming another pig-man, running 100 blocks away out of range, etc... does not seem to calm pig-men.
remaining in chunks for hours , or leaving chunks for hours and re-entering after the chunk unloads does not calm the pig-men.

Anger Effect possibly is global on all pig-men instead of just near-by pigmen.

server restart remembers that pig-men are angry.

pig-men continue to spawn angry.

switching to peaceful, then switching the server back to PVE / PVP -
the pigmen stop being angry during peaceful but remember they are angry when the server is switched back.

Server admin had to clear ALL angry pig-men in ALL effected chunks using server commands in a spectator mode to stop the rage from spreading.

Issue has appeared on some forums by other unknown users:
https://www.minecraftforum.net/forums/minecraft-java-edition/discussion/2962813-problem-with-zombie-pigmen