After some experimentation, I was unable to craft a test that caused the non-deterministic ordering to cause a problem with returned voters. I think this is because of the way the query is crafted and because of how the poll votes are created, but I am not 100% certain.
I’ve created a PR with my test and the fixes I propose: FIX: Avoid returning duplicate voters from `/polls/voters.json` endpoint by clechasseur · Pull Request #34433 · discourse/discourse