MacOS clients now (unsure of exact version introducing behavior) prohibit guest sessions unless the user explicitly selects that they want to perform guest authentication. When guest access is enabled in FreeNAS, the server will remap "bad user" (i.e. user does not exist on the server) into the guest user account and flag the session as a guest session. The new-ish behavior from MacOS is to reject SMB sessions that get guest access when they wanted real access. This means that if MacOS clients must explicitly select "guest" when trying to access a public / guest share. MacOS does not provide a clear error message in this case (only visible in Console).