I don't think this is a bug, but rather they did it fully on purpose.
The diamond, in this case, is not relevant. For Community Wiki posts, the author is not really relevant, so there are also no other details, just the name. No reputation, no badges, no user card.
If someone wants to see details about the author(s) of a wiki post, they can just click the link and see the full revisions list, where the diamond is showing just fine:
Note that unlike ordinary posts, even without any edits, the author name is link to revisions, so they are always one click away, no need to manually change URL like in the case of ordinary post without edits.