Sounds like the security features of vote by mail are working as designed. A very small race like this is the only scenario where someone might bother to attack it this way, and there are thousands of them, so if that's what this is, I guess it's not surprising to see it happen somewhere.
Also, why is this on HN?
Remote voting has become a news item in the USA recently, partially because of Covid. And voting theory is pretty interesting - in this instance it seems like a classical security vs convenience trade-off.
The tradition of secret ballots goes back a long way but maybe with almost ubiquitous internet we've reached a point where it's no longer worth the cost.