When users are registered through the front-end M2 Registration form, their username is not shown at the bottom logout link in account page.
Use M2 Register shortcode, or go to registration from M2 login page. Or from Memberships page, just register a user account through front-end.
Then go to the account page (e.g. site.com/account/)
At the bottom of the account page, there is a logout link. It doesn't show the username: https://monosnap.com/file/tfIUlC2gTcSJW5wA642HKfYigjyWLe