Yes the modification is not carried over and the cross-post will often have a different author from the original too. While creating the cross-post the cross-poster could have edited the title and description, too. The only connection is that they have the same url (in the attachment). Lemmy adds the text "cross-posted from link-to-original" to the beginning of the body content but it's just text, not an attribute on the Activity.
Room for improvement!
Also moving what NodeBB calls a topic (a post in Lemmy/PieFed) from what NodeBB calls a category (community in Lemmy/PieFed) into a different category (without spawning a new topic or leaving the old copy behind) is much needed but not implemented in Lemmy/PieFed/Mbin.
