Updating user email does not update username when useEmailAsUsername is set

When the config setting useEmailAsUsername is set, I would expect updating a users’ email address to also update the username field when saving the element. However, this does not occur unless you manually call php craft utils/update-usernames.

Should the username be updated when a users’ email address is updated?

Is there a method to trigger this for the current user via a plugin or do I just need to update username alongside email?

Version: 3.5.15.1

Edit: This also prevents registration of a new user with the previous email address but does not indicate what the problem is clearly.