Mention event + Post context

We're using POST /v1/inbox/mentions/reply for Instagram. The docs(earlier it said..maybe now removed after flagging) say to source mediaId and commentId from Instagram's comments webhook — but comment.received only fires for comments on our own tracked posts, not when our handle is @mentioned on someone else's post. How are we supposed toreceive Instagram mention notifications? Is there a webhook event or endpoint we're missing, or is Instagram mention detection not yet supported?

Also — for a post we don't own, is there any endpoint to fetch post metadata (caption, image, permalink) given a mediaId? We see the post object in the comment.received webhook payload carries this, but since that webhook doesn't fire for mentions on foreign posts, we have no way to get that context either. Does GET /v1/inbox/comments/{mediaId} or any other endpoint return post details for content we don't own?