I get this regularly, even though I am always providing Amazon with original documents from the supplier. I have had this when dealing with ungating as well as with authenticity complaints.

When it most recently came up, I called up the account health team asked why, and told them whatever software they are using to determine authenticity is not working correctly. They said my documents were being flagged as unauthentic because the 'date modified' on them was after the date on the invoice. This is insane. If you download a PDF from your email, the date modified becomes when you downloaded it, not when the invoice was generated. I told them to try it and see for themselves.

Maybe the person I spoke to was misinformed about how they check these things, but I'm genuinely stunned that the account health team is so stupid as to make this a criteria for determining authenticity. Getting authenticity claims removed from your account is incredibly important, and so the fact that they don't understand basic facts about how computers work is incredibly frustrating. The only way I've been able to get past these issues is to keep re-submitting the same document over and over until someone with a brain looks at it and sees that it is an original document. Sometimes it takes half a dozen submissions. You can also call them, they tend to be a bit better on the phone. Good luck.