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.
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.
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