Also for @eating3645@lemmy.world

In the Settings -> General page there is a 'Show EVERYTHING feed' toggle which could explain why you're getting that problem. That gives an option in the drop down at the top of the main feed page to view All from Everything which means it combines the All feeds from each instance you have in Accounts in the Settings (by default it has a few of the big instances added I think). It's basically like you're going to each site beehaw.org, lemmy.world, lemmy.ml, etc. even if you don't an account there.

To avoid running into the issue of not being able to upvote you can remove the instances you don't have accounts on, or just not use the Everything feed (turn it off, or select the instance you want to view from in the drop-down). And in case it's not clear, if you view All from just your one instance you'll still see posts from everywhere else but you are seeing it from the place you have an account so you'll be able to upvote/comment.