Hmm... I think I found something.
I got these sets from the same person: GER SUM2011 (I have two sets, got another set from a different person), SWISS SO2011, SPA VER2011, and FRA ETE2011. I've definitely looked at all of them before because I traded out the VER2011 set to my friend, and I remember only the Spanish set giving me an error in PKHeX. Now, I'm seeing everything by this person giving me an error.
Apparently my French set's OT is ETE2011 when it's supposed to be ÉTÉ2011. It's weird because PKHeX has never given that an error before. That means my French set is definitely hacked. SO2011 gives an error which is normal because PP doesn't have the wondercard for it, and it's been contested back then about whether or not this event truly happened in Switzerland. The German and Spanish set given to me by this person all say Invalid Ribbon: Souvenir Ribbon. Now that I know my French set is hacked, I'm inclined to believe they're all hacked.
This person also gave me full characteristic sets of the WIN2011 Beasts (Raikou, Entei, and Suicune), but that's not odd because back then, characteristic sets obtained by wondercards were in demand and were a commodity.
So now that I know my extra ribbon error is hacked, I'd say yours is hacked too. Bulbapedia says Premier Ribbons in gen 5 should only be held by Event Mewtwos, and that's the only ribbon it should have. If a Premier Ribbon is on a gen 5 event Pokemon that's not a Mewtwo, then it's most likely hacked.