Not nessasarily, the protocol could be written so that an instance simply tells other federared instances “X of my users upvoted this, and Y downvoted this”.

The tradeoff being that instance then have less tools to work with to moderate voting. Instead of being able to do global vote ring detection, the most they can do is look for abuse on their own server, and trust that every instance they vote-federate with does the same. Even then, with every instance trying to be vigilant, no one instance would have the info to detect a cross-instance abuse.