For some weird reason in the implementation of the AP protocol, lemmy posts are seems as just a link on mastodon, the replies are complete though.

ActivityPub allows two post formats, Notes and Articles. Articles support titles and therefore posts on Lemmy and Threads on /kbin use them, while notes do not and are therefore used for microblogging and commenting. Currently Mastodon's article federation only goes so far as linking the post for content, and to be honest I'm doubtful whether Threads will federate Articles at all given their carefulness with federation.