Being in the design world since a while as art director, I can tell you that Fiverr has a terrible reputation and that no art director in his sane mind would go there to hire anybody. - it’s almost a running joke.
It is famigerate for „portfolio stealing“, delivering stock art and design and claiming it to be original work, and „designers“ plagiarizing other people`s work....as weil as, of course, for ridiculously inappropriate prices and practices.
The site survives because it appeals to a certain type of client you would not want to work with anyhow.
There are many sites with a similar setup, and literally millions of freelancers on them. The majority of portfolios on these sites are not google-searchable
(It´s just a setting you can choose when you register there as a freelancer), so there is no way to know how many are using other people’s art as a hook. I hire production designers and other type of low-creativity jobs on Upwork and I always run a check when I see an unusually good portfolio. Twice I found the person on Upwork was claiming somebody else`s portfolio as their own.
The bottom line, I guess, is that I would not worry too much. Of course it sucks, but this type of deception is factually not fully detectable, has very short legs and does not really damage your business - not more than what sites like Upwork as a whole do already since years.
It´s much more damaging not to put your work out there for people to see...