It appears that, beginning in 14w06a, Zombie Pigmen will now stop being angry after 20-40 seconds and lose their aggro speed boost, although they will continue attacking the player until the player leaves their aggro distance.

But in 14w07a, baby Zombie Pigmen will no longer forgive (possibly a side effect of MC-47817 having them no longer get the aggro speed boost).

It's not clear to me whether the bug is that adults forgive or that babies don't. But it seems like they should both be the same in this.

To reproduce:

Spawn one or more adult zombie pigmen. Hit one, wait 40 seconds, then run away and return (or quit and reload). You won't get attacked.

Spawn one or more baby zombie pigmen. Hit one, wait some time, then run away and return (or quit and reload). You will get attacked.

Note that, if trying to reproduce with both adult and baby zombie pigmen present, the babies will re-anger the adults when retargeting the player.