Portals; Security page; "Web User Id is Email for Password Reset and MFA" not working as expected

On the Security page of the Portals form, if the Authentication method is "Client" then you are given the option, towards the bottom of the page, to select the Sign-In Method.

To the right of this dropdown is a checkbox "Web User Id is Email for Password Reset and MFA?".

This checkbox is not applicable if the Sign-In Method is set to "Client" but was not hidden until the record was saved.

Also, the value of this checkbox was not cleared in the database when the Sign-In Method was set to "Client" meaning that any Password Reset or MFA Emails sent to the Client would use the Client's Web User Id in preference to the Email Address shown on the Client's Contacts page.