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.
Okay, that’s good to know. It’s definitely confusing to me. I would expect the post preview to always match the final post.