From Bulbanews, your community Pokémon newspaper.
Affects winner in tie-breaker circumstances
Normally, in a situation where there are only two Pokémon remaining on the battlefield and one of them is knocked out by an attack but the attacker is also knocked out from the damage inflicted by Rocky Helmet, the player with the Pokémon that was holding the Rocky Helmet would lose the battle. Currently, the player with the Pokémon holding Rocky Helmet will win the battle.
Note that in previous generations, the Pokémon holding the Rocky Helmet would win the battle. In previous generations, if both Pokémon fainted due to the Ability Rough Skin, Aftermath, Liquid Ooze, or Iron Barbs, the Pokémon with the Ability would win; in Pokémon Sun and Moon, the Pokémon with the Ability will lose.
Since this only occurs in tie-breaker circumstances, this should have no impact on most battles involving a Rocky Helmet. The item is still legal for use in online play and tournaments.
This discovery follows Parting Shot and Memento being banned from Battle Spot due to issues related to those moves in Pokémon Sun and Moon.