ska can you please check that those three posts are actually federated outwards?
The human-readable URLs don't contain the expected Link header to the ActivityPub resource.
e.g.
link: ; rel="friends-base-url"
link: ; rel="https://api.w.org/"
link: ; rel="alternate"; title="JSON"; type="application/json"
link: ; rel=shortlink
It does contain the shortlink, which I tried to pull with the appropriate Accept header, but was sent HTML in response.
Perhaps there is a toggle you can press to enable federation on those posts?