Custom Membership Field not populating in membership form

Integer (radio button) custom field (Used For = Membership) in an included profile for membership signup and renewal doesn't populate the field with the member's existing value when viewing the sign-up form (Main.php).

• If I look at the member's information, I see the field correctly in the "View Membership" page.

• When I view the sign-up form, that member's other information fields are corretly populated. The custom field shows up as a series of radio buttons, but none are selected.

NOTE: PostProcess / confirmation page display etc. all work fine.