TL;DR: this is a non-starter of an issue because the only people who have any ability or desire to resolve it are the artist and commissioner themselves. Here's why...
Artists do hold copyright to the specific work they do, if they are not signatory to a work-made-for-hire contract, which must be signed and cannot be implied.
That said, any artist who claims that a contracted price for art is a "license" which can be revoked would lose in court and open themselves to countercharges of fraud unless they specified - also in writing, and also signed by the receiving party - that a license existed, as well as what its specific terms are.
Moreover, use of a sold work (even without a WMFH contract) without alteration does not amount to a copyright infringement in any event. If you draw someone else's character, and they use that image in a post where they talk politics you don't like, that's not a "derived work". Nor is it derivation if they alter a work they paid you for to be more to their liking. That is, in fact, simply "speech" and unless it's being used in a commercial fashion to bring in substantial monies - we're talking like $75,000 and up - it means nothing to any US District Court, which is where you'd have to sue.
Similarly, no artist receives any copyright in original content which they are creating new content FOR, only in what they themselves create.