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.