What you show should work as long as the records are correctly sorted.
Are you sure that your records are really sorted by Cabin and not some other fields such as "cabin ID"?
You nailed it...the sort field in the sub-summary part is <<_ID Cabin fk>> not the primary key field. I made the change and it is now functioning properly.
Note that if necessary, you can sort on both fields in order to get an order based on ID, but with the number series restarting.