There are rumours surrounding USDT since a few years now and they are just not going away. The reason for that is, that the company behind USDT changed the wording regarding their backing of every USDT minted. In the beginning they always said, that they have enough real USD to back every USDT minted, but now they are saying that they have enough reserves to back every USDT minted. The problem is, that no one knows if those reserves really exist and in what form they exist.
I think certik and fairyproof only audit the code and not stuff like reserves.
This may be explained by the fact that part of the reserves are bitcoins that were purchased for USDT. I can assume that other top coins may also be stored as reserves. But these are just assumptions and due to the fact that the Circulating Supply USDT is about $80 billion, I would like to see more open information about what exactly provides such a large amount of stablecoin.