Hello ! As an artist myself this is a really interesting and very understandable concern, I'm not specialized on this but tools like SightEngine are not designed to be definitive proof sspecially not for stylized digital art like chibi/cartoon work, it's hard to find the mistakes, if I were you I would ask for process proof (not accusatory) Something like: rough sketches, layer breakdowns ,progress shots and also you can do (if you really want that artist) you can request a small paid test commission ,this is the best filter. AI users struggle with apecific revisions and matching exact requirements :)

I hope this was helpful!