The solution to this was that prior to iOS 18 if you had a one to many relationship in your SwiftData models that adding to the array in the "one" data model entity would automatically update the "many" data item so that its optional attribute that pointed back to the data item containing the array of "many" items when you added items to the array. In iOS 18 now this is no longer true and if you don't set that optional attribute to explicitly point to the data model that will contain the array of "many" items then you will see the aforementioned error when you try to append the new item to that array. Hopefully this makes sense and will help someone down the road.

That is a very nice tool and thank you for that! I beat my head against this issue for a couple of days... trying everything I could think of in the PrivacyInfo ... and lo and behold I was creating the plist correctly... but for some reason if you add the PrivacyINfo into your xCode project, unlike adding any other Swift file, it doesn't mark the PrivacyInfo file for Target Membership in your build. So as you continue to submit trying to figure out why you keep getting those emails from Apple ... this may be the reason. Check that the Target Membership was selected for this file in xCode...