I've since bought S/M, and after catching and researching with Minior, I find this answer correct. Your comment that they are always meteor form outside of battle is incorrect. Sure, they first appear in wild battles in meteor form, but they are in fact core form outside of battle (i.e. in your party or box), regardless of their HP at the time. As contradictive as it sounds, without the ability available to deactivate, it will remain in its starting form, which is core.