This instance has to first been made aware of the magazine, which happens, when someone searches for the url at https://lemmynsfw.com/communities. After that, it takes a moment for the magazine to be added to the database. I had a similar experience, see https://lemmynsfw.com/comment/76851
I had the same experience. Writing something like lemmynsfw_dot_com so that it isn’t recognized as an URL (still) works.