I'm not 100% sure this is correct because I haven't looked at the protocol or implementations in detail yet, but I can offer some educated guesses.
"Pending" implies that you're waiting for some remote service, likely the other instance, to confirm that the subscription went through.
I can think of a few reasons why an instance may never send confirmation or why the message may not update:
that instance defederated your instance, meaning you can see them, but they refuse to see you. This makes sense for beehaw, given that they've defederated several instances, including lemmy.world
that instance hit an error
the request or response was lost somewhere along the network
your client, that is, the website or mobile app you're using, has a cached state of pending and never updated it even though it received confirmation
Maybe I'll dive into things tonight and get more info. Or i might forget. We'll see 😅