The missing username field might be related to Easier account signup using email codes. Is that change enabled on your forum?
The description for most default fields was removed about 2 years ago in UX: Registration design and validation improvements - Pull Request #27666 - discourse/discourse - GitHub. After the feedback in Recent changes to the signup form, settings have been added for showing the descriptions again. You can find them by searching the site settings for instructions. The description for the name field is, for example, controlled by the show_signup_form_full_name_instructions setting. Is that enabled? Then I would try disabling the upcoming change.