@nikclayton@mastodon.social sort of. It's not anything NodeBB is doing explicitly. Mastodon searches the note content for links, any links it can find, and turns them into a preview.

Specifically, Mastodon won't do it to mentions if you have some specific classes attached to the anchor (microformats). NodeBB doesn't do this, so Mastodon doesn't ignore it.

It's rather annoying behaviour on behalf of Mastodon, and one I'm not keen to bend over backwards to fix.

It'd be better form if Mastodon explicitly listened to the attachment property, as specified by the ActivityPub specification instead of departing from it in an odd way.