From what I can observe lurking, communication between Lemmy and Mastodon is pretty half-baked, with things overall boiling down to users on Mastodon being able to follow Lemmy users, but not the opposite, and posts from Mastodon users may appear on Lemmy but with the text body appearing as title instead. Issues like these may appear due to how each site engine handles the data received. If you must follow users from Mastodon on Reddit-like forums, Mbin is far more compatible.