Pretty sure this has been brought up in the past and is working as they intended it to. It looks like that for all connected accounts on my site.
Maybe XF could save information like the remote username locally to not have it show up as "unknown" instead? That way it wouldn't look broken to the user at least, even though the information might be outdated.
It does indeed take a while. Fwiw, it is able to keep some elements for certain providers, but not all of them. I've added all my connected accounts about 6 months ago, Facebook, Twitter & Google, as well as my self-written DeviantArt, Discord & Twitch providers. Of all of them, Facebook has retained the avatar and twitter avatar and profile id. The rest of the providers shows up as "Unknown account". The facebook "Unknown account" links to an invalid profile ID.
As mentioned above, this is mostly expected and it's not something we're looking to change right now. There are potentially other approaches we could take in the future, including making subsequent API calls to fetch the most recent account data, or caching the original values (for the name, at least) but for now we've changed the phrases in use here to "View account" if a link to the provider's profile is available or "Account associated" if no link is available.
FWIW the behaviour was exactly the same in XF1 too so it is not something which has presented itself as a significant issue over the last several years.