Found an oddity that I would like to know if it is intentional behavior, or a bug?
When you have a parent and child table, with the child ref-ing back to the parent and ispartof=true. Then you go and edit the parent record, and it shows a list of child records. If you select ‘new’ from that inline view, it automatically links the new child record to the parent record. However if you first select ‘view’ from the inline view, and then hit the ‘add’ button, it does NOT automatically link the parent records.
I imagine this is so because technically you are viewing a filtered set of ALL records of the entire child table, and so the ‘add’ button will add any record to the child table, not just one associated with the parent.
My feedback, if this is intentional behavior, is that this is not very intuitive. Since the user is viewing all child records associated with the parent, it would make sense that adding a record here would also automatically associate it with the parent. In my specific case, the parent ref column is marked as required, but also hidden (since it is a uniqueid() nonsense string), but even though it is required, it will allow a child record to be created without any reference.