It's not a bug, just bad UX. Liftoff aggregates content from multiple instances on its own, rather than viewing other instances "through" the one you're logged into. It allows you to log into multiple accounts at once to compensate, but it's definitely not how fediverse sites are "supposed" to work.
IMO, all liftoff needs to do is check if the post you're viewing is from an instance that federates with an instance you have an account on. If it is, then it should automatically route your comment "through" the instance you have an account with, and make that transparent to the user.
Your user login only works on your instance. Any instance that is federated with your instance you can interact with through your instance. You cant interact with a different instance directly.
Which is why in the screenshots above You cant vote on the first one since that is interacting directly with lemmy.ml. However you can interact with the second because you are interacting with lemmy.ml through lemmy.world
That's because liftoff aggregates posts from multiple instances on its own, rather than relying on instances "federating" with each other. The benefit of this is that it gives you control of what instances you see content from, rather than the instance owners deciding for you.
Personally, I think that makes way more sense than making instances "federate" with each other to share content. The only interoperability instances really need is the ability to post to communities that are hosted on other instances. We don't need instances to be sending the entirety of all their content to each other; clients can aggregate that on their own.
At the same time, federation allows citizen administrators to create and maintain new instances. Let us say there are 1000 instances with 1000 users each. It is relatively easy to host a server for 1000 people that federates with max 999 instances. And yet, one post can reach 1 million people.
The same would be much harder for a private person and 1 instance that has to serve 1 million people.