The problem with the Mienshao is actually not itself, but Wonder Trade. Pokemon migrated from Gen 5 or earlier to Sun/Moon are banned, due to different filters concerning moves. The level and TM's of moves learned by the same Pokemon from different generations may be different; and you can check Mienshao's data in this website itself, and you'll see it's different.
It's not only Mienshao, but if you compare the moves learned of a Pokemon from Gen 5 or earlier, and the same Pokemon's moves for Sun/Moon, chances are they are different. So, when you wonder traded, the person who you traded it with could of transferred it from Gen 5 - and this is most likely the problem. Battle spot usually only allows Pokemon in-game to make sure the growth of the Pokemon is legit, and not altered or hacked.
Otherwise, it's definitely the dream ball
Hope I helped! :)