Existing subform data is being changed when new subform entries are added

I'm having trouble with existing subform data being changed when new subform entries are created.

I have the following setup to track registrations for a girl scout troop:

Main Form: Child
Subform: Registrations

The data are a one-to-many relationship where each Child record has many Registrations (new Registration will be created for each year the child is in the troop.)

Per the instructions, I have created the subfom, added it to the main form, gone back to the subform and created the bi-directional lookup field based on the EXISTING relationship.

With this setup I am able to add a new Child record (main form) and at the same time create a Registration record (or even multiple records) in the subform.

I can verify that all data is saved successfully when I view the reports for the main form and subform.

HOWEVER, when I edit an existing Child record (main form) and then try to add a new Registration record to the subform (within the main form view) all previously entered subform entries for that record are changed (they are unlinked from the Child's record).

Here's the result. Notice that only the Registration record I added last remains linked to the Child. The previous records are unlinked but remain in the Registration dataset.

In order to make sure there wasn't something wrong with my forms, I created another simplified form/subform pair following these instructions and I get the same behavior.

Can anyone help me understand why this is occuring and come up with a solution to preserve original subform records after adding/updating a record?