cid=0 contribution form overwrites contact information for admin filling out form

If I fill out a front-end membership contribution form with cid=0 in the URL and a new, unique name and email address, I get different behavior depending on the permissions granted to the user filling out the form.

If I fill it out from an ordinary user's account (with minimal permissions), it creates a new contact and membership with the new information, which is the correct behavior.

However, if I fill out the form from an administrative account that has sufficient privileges that the "select someone else" drop-down menu is visible (which I leave blank), then the new information does not create a new contact; instead, it overwrites the contact information and membership of the administrative account.