Have you tried logging-in using the account which is associated with the email address on the profile and then making the changes (I assume this would mean logging-in using the project account not your own account)?
I would guess that the project account profile is treated like the profile for any other active WikiTree user. These profiles have special rules regarding the managers of the profile (see https://www.wikitree.com/wiki/Help:Profile_Manager#Profile_managers_for_active_user_profiles), so I suppose this is just one of those "special rules".