After a lot of testing, I have found that the actual bug is that angered Pigmen do not spread anger to newly spawned Pigman when the angered Pigman is less than ~34 blocks from the player.
If you set up this same scenario, you will find that the new Pigmen will become angered when you move far enough away from the already angry Pigman (as long as it does not despawn.)
Alternatively, if you trap and name another Pigman more than ~34 and less than ~40 blocks away from the player (make sure it still has line of sight) you will see that the newly spawned Pigman do become angered as you would expect them to. No need to strike the already angered Pigman again.
Dennis Urie
After a lot of testing, I have found that the actual bug is that angered Pigmen do not spread anger to newly spawned Pigman when the angered Pigman is less than ~34 blocks from the player.
If you set up this same scenario, you will find that the new Pigmen will become angered when you move far enough away from the already angry Pigman (as long as it does not despawn.)
Alternatively, if you trap and name another Pigman more than ~34 and less than ~40 blocks away from the player (make sure it still has line of sight) you will see that the newly spawned Pigman do become angered as you would expect them to. No need to strike the already angered Pigman again.