I'm an event collector of sorts and I only use PkHEX to inspect things that I've received in trades.
I was traded a Shiny Shaymin from the Oak's Letter event from a Japanese contact I know. Basically, he said that he found the Oak's Letter in a used cart that he got and he SRed for the Shaymin himself. He traded it to me, and after inspection, it comes up as 'Invalid: Encounter Type PID mismatch' and 'Invalid: PID-Nature mismatch'.
Is this a guaranteed hack? I mentioned this to him and he was adamant that he really did SR for it himself but said that he obviously couldn't vouch for the item itself, which was just in a used cart. He says he genuinely doesn't know what the deal is, but offered me a tradeback if I want it. I told him to hold off for now while I investigate, basically.
My question is basically this: Is there any way that this flag can come up incorrectly? Also, can this flag come up if the Oak's Letter item was hacked in but the Shaymin was encountered and SRed for normally? Or is he just lying and it 100% means that the Shaymin was hacked?
I'd appreciate responses only from people who are quite sure of what they're talking about (not just 'shiny shaymin = hack'), since I know the guy fairly well, although I do acknowledge the possibility that he's just lying.
Thanks in advance.