Though the weekly WEEKLY_INCIDENT_ACTUALS hours act like an internal slice it is actually not, and the Start Date needs to be Monday on this slice just as the internal slices are set. Not the internal slices are systematically set and cannot be changed to a different start day.
Resolution
Make sure the WEEKLY_INCIDENT_ACTUALS slice starts on a Monday and covers the range you need.
It has been observed that this slice as well as the other Incident Actuals slices start near current period versus in the past. If you are using the INCIDENT_ACTUALs slices make sure they start far enough in the past to provide the actuals you need.