Subscribing to ESR switches partner membership to regular member
Ex: https://civicrm.org/civicrm/contact/view?reset=1&cid=16310
Regular Membership and Partners are linked to the same org, so the membership is considered an "upsell", i.e. it updates the existing membership.
cc @josh fyi I don't know how many have been affected. Should be possible to detect in the civicrm_membership_log table.