CiviCRM 4.4.4 invoice email address overwriting primary email address

When a new user signs up with a membership they are able to enter their primary email address when they define their personal details. If they then choose the invoicing option for payment they are provided with the opportunity to follow a link and enter separate invoicing details i.e. for a CEO or a PA. This detail editing page displays the "primary" email address as an un-editable field for reference, and also a field that allows the user to enter an invoicing email address which is set to "invoice" type. Both email addresses should then be stored in separate fields, however when save is pressed, the invoice email address overwrites the primary email address which is incorrect. This functionality doesn't seem to happen when the person is signed in already and then chooses to amend the details, it appears only for new users.