This is an old issue but not on the bugtracker as I can find. Zombie Pigmen equipped with a helmet in the Overworld lose their helmet like zombies and skeletons do after a period of time in direct sunlight, even though they do not take damage from the sun like other mobs do. The sound for the equipment breaking even plays as it normally would for other mobs.

Reproduce: drop a helmet to a Zombie Pigman in the Overworld in daylight, and wait.