Yeah, actually it's on purpose, with this known downside. For link posts in our forums, we put the link in the post's url field, the same way Lemmy does. We did this as a compromise, because otherwise NodeBB wouldn't show the link. The side effect is that Mastodon-style apps treat url as the post's own address. "Open original page" then opens the link instead of the post on the Catodon server.
@julian@activitypub.space - hello! I have not checked if NodeBB has changed this since we implemented federated forums. if we send url as a list, with the post's own address first and the link second, would NodeBB still show the link? We also send the link as a Link attachment. Ideally, we would like to keep NodeBB working and fix Mastodon too.