The above link shows up strangely in post previews. I’ve seen this numerous times over the last few weeks. It shows up correctly (as a raw link) in the actual post.
The notice you are seeing in the preview window is telling you why a onebox cannot be generated for the link. You should only see this behaviour if you paste the link onto its own line in the composer - the same way as you would if you were trying to create a onebox for the link. The goal with this is to let the user know why a onebox is not being generated.
What you are seeing is the expected behaviour, but knowing that the expected behaviour is causing confusion is good feedback.