You cannot make votes completely private, one instance has to have the authority over which votes do exist. This instance should be the origin of a post or comment.
At the moment it works like this: you upvote a post, this upvote gets send to the author of said post AND the magazine and that magazine then broadcasts your upvote to all subscribers of said magazine.
I could imagine that the process looks a lot different: you upvote a post, this upvote gets send to the author of said post, the author of the post then sends an update to the magazine saying how many people have now upvoted their post and the magazine then broadcasts this info to every subscriber of the magazine.
With that you would of course have new limitations concerning moderation and maybe there are trust issues regarding the correct reporting of that upvote count, but only the author of the post (and their instance ofc) could technically know who upvoted their post. As in everything here this is a compromise and whether the gained privacy is worth the other limitations, I don't know