Beehaw defederated from Lemmy.world, but Lemmy.world did not defederate in return.
Instances copy content from other instances. When thousands of Lemmy.world users want to read a post from a tiny instance, they can. All that traffic is served from Lemmy.world’s local copy, and the tiny instance is only bothered by one download request from the Lemmy.world server.
When Lemmy.world asks beehaw for posts to update its local copy of a community, beehaw agrees and sends the content. So Lemmy.world users can see new beehaw posts when they connect to their home instance.
Before defederation, beehaw would do the same. Beehaw asks Lemmy.world for the latest content, and beehaw updates its local copy.
Once the defedeation started, beehaw stopped asking Lemmy.world for new content. But if a beehaw user tried view a Lemmy.world community, beehaw would happily send its local copy, blissfully unaware it was out of date.
Lemmy needs better user-facing error messages. This is too confusing for normal users, but its reasonable behavior for pre-alpha software which is what lemmy is right now.