I think the commissioner is well within their rights to transfer the private-use 'ownership' of the work to other people
morally sure, but legally no. they can pretty much only do with it what the artists say they can (in addition to use that doesn't infringe on the copyright in the first place obviously), and I don't know that I've ever seen an artist actually include that in their "terms of service"
not that big of an issue I guess, but it means it's just completely unenforceable from the perspective of the buyer of the character. the original "owner" could still do whatever they wanted with the character and art and nobody would really be able to do anything. FA might remove it but.... wouldn't count on it lol