Edit: obligatory explanation (thanks mods for squaring me away)…
What you see via the UI isn’t “all that exists”. Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see “under the hood”. Any instance admin, proper or rogue, gets a ton of information that users won’t normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.
Edit: To clarify, not just YOUR instance admin gets this info. This is ANY instance admin across the Fediverse.
I would hope this would be obvious to anyone. If your client can highlight which posts you have upvoted in the web and app UI then the fact that your user specifically upvoted that post must be recoverable from the instance server and thus must be recoverable by the instance admins. I would not expect anything different.
I also was not expecting how Lemmy is different in that way. I was expecting it to be only accessible to the admins of a user’s instance.
But some people would make that assumption, which makes this post worthwhile I think