An account on one instance lets you interact with all other instances, but only via that instance, not directly. You are probably viewing the “All” feed via an instance you are not signed into. Try this:

Click on the top button:

https://i.imgur.com/0PEHYsK.jpg

Select “all” for the instance you are logged in for, for example I am logged in via Lemmy.fmhy.ml:

https://i.imgur.com/RSGL4y5.jpg

Just ran into the same problem. I fixed it by going to settings -> accounts and them pressing and holding on each instance that I don’t have an account on and removing it. Then it uses the instance I do have an account on.

There is a difference with adding an instance and accessing it that way (via their API - bypassing the server you signed up on) and via federation using the server you signed up on. Only in the latter case can you vote using the account already registered.

Try using the search feature to access the community via federation (e.g. “!cats@lemmy.world”) rather than adding the instance directly.

It probably means that you have enabled the “everything feed” in liftoff settings. This aggregates content from all instances apparently, even those not federated with yours.

Edit: weird cause you seem to be on sh.itjust.works which is federated with Lemmy.world.