I'm confused on your question.
Are you saying the post only has one upvote (I see 14 up and 1 down?
Or are you saying your database only shows them as having used 1 vote? It's a "daily" vote limiter so maybe it reset? Also, they aren't on your instance so you may not be federated with everywhere they used their votes.
Or are you asking something else?

No worries. Appreciate the clarification.
That does sound weird. Could be a different account. I believe the servers only store the last like 3000 votes and drop the older ones (my votes last about a week). So, maybe something with that. Maybe a Piefed.social admin removed them because they considered them invalid or wasteful.
I think we'd need to ask a piefed admin or OP to get a better idea of what happened and why.