I got the answer.
It's the Heal Ribbon. In exchange for faster HP regeneration, my Shaymin lost it's belly stat and it went to zero, making it get the "Hungry Pal" status. Upon infliction of this status, the damage of all moves used is reduced to 1, and the partner is immobile. To prevent this event occuring, the Heal Ribbon can be removed, and thus will not affect the Pokemon, same case as Munchlax and Snorlax's "Munch Belt".