Why can't I set the email address in an AAD user profile?

Question

I have an AAD tenancy with three accounts including two global administrators for safety reasons as recommended ("Administrator A" and "Administrator B"). A was verified by email some time ago and B has yet to be verified (I've been busy with other things). I see that the profile for A has an empty "Email" field and a populated "Alternate email" field.
I wish to populate B's "Email" field. From the profile, I click the Edit (pencil) icon at the top of the page; this gives me a page with lots of editable textboxes, including "Email" and "Alternate email" - see below.

Whenever I enter the email address for B into the Email field, the form seems happy - it even puts a green tick beside it. But when I click Save, it wipes it again and gives the error message "Email is required" (see below).

I also notice that the field "Alternate email" has an "Edit" link above it. This seems bizarre because the field looks as though it is already an editable textbox, but when I click into the field I find that in reality it is not editable. Maybe this is how I ended up having an "Alternate email" for A but no "Email" - I don't remember now. Certainly when I try populating "Alternate email" via its Edit link I find that I am able to do so (although the user interface for doing so is a bit weird), so now I have B configured in a similar manner to A and hopefully that will allow me to verify the account by email.

Does anyone have any clue as to what is going on? Why can't I enter an email address in "Email" but I can in "Alternate email", and why does "Alternate email" look as though it is an editable textbox on the Edit form when it is no such thing?