Maybe this is a front end thing, going to user profile the url is /u/user@instance and my /u/ format creates a link for me but the @ format doesn't.

@rand_alpha19@moist.catsweat.com @DrunkEngineer@lemmy.world @theluddite@lemmy.ml @spacesatan@lazysoci.al This one generated a mention in my fediverse server, for what it's worth.