When an Event without a date is added to a Series, its automatic order is #1. After the same Event is edited to have a date, its order in the Series is not updated to place it in the correct spot. (The same issue occurs if an existing Event date is modified to a degree that would affect the automatic sorting.)

If the Series is then opened for editing, the events will sort correctly, but it's not possible to submit an edit that forces the updated sorting to apply unless a new Event is added to the Series.