that looks like mastodon adding a embed link preview because the links class doesn't match what they use. i think the fix is at their side since me mentioning people in lemmy works fine.
@Saiwal Mastodon has their own alteration of AP, this might be one of thpse cases, where they are just not acting standard-compliant. @Mario Vavti will know more.
@Saiwal would you be willing to explain me how mentions work with activityPub, as you seem to have some idea of it ? (don't bother vulgarizing, I worked in IT as a dev, even if I burned out)