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.
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.
It is be because all feed includes Lemmy.world, Lemmy.ml and beehaw.org. That means there will be duplicate posts retrieved from other instances. If there is a Lemmy.world post they would see it 3 times: one from original Lemmy.world version, one from Lemmy.ml and one from sh.itjust.works. Bit can only comment on sh.itjust.works version because they are not logged into Lemmy.world or Lemmy.ml
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. "[email protected]") rather than adding the instance directly.
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: