So, I set a respawn anchor, then hit a zombie pigman. After I was killed, I respawned, then they kept attacking me.
1. Go to the nether, and set a respawn anchor
2. Hit a Zombie Pigman
3. Get killed by it.
4. Enter the Nether again.
5. The Pigman will keep attacking.
The video was too large, so, YouTube: https://youtu.be/S4E1Mj1un-Y