I believe this is "intended behavior", though more warning messages really need to be made.

beehaw.org has defederated from lemmy.world. That means that you cannot post to any community on beehaw.org (at least, not until they refederate).

I think the developers need to have servers detect the one-way federation situation (ie: lemmy.world still wants to federate with Beehaw.org, so lemmy.world is still importing all the posts/comments from Beehaw.org... but Beehaw.org has blocked lemmy.world from their end, so posts/comments will NOT go through). Then, when this situation is detected, tell users "Warning, your posts will not go through because Beehaw.org has defederated from Lemmy.world".