Pay-later membership payment overwrites previous record

1. A contact has an existing Membership Record for a Membership Type (#1). I'm Not sure if this is relevant, but it was paid by check; i.e., the Membership record was created after an admin changed the Contribution status to 'Complete'

2. The contact completed a another Membership Sign-up Contribution form for another Membership Type (#2) also using the Pay later option. At this point, you can inspect the civicrm_membership_payment table and see that the Membership ID recorded is for the Contact's #1 membership, not the new membership!

3. So when the admin marks the payment complete for Contribution #2, instead of creating a new Membership, the original Membership is overwritten, changing it to type #2, though some details (like "Source") from the original membership are preserved.