Hi @Anonymous ,
May I ask if your datacard called ‘Privacy/ATIP Manager Approval' is a form built with the SP list as the data source?
We assume that your datacard exists in a form with DOA as the data source.
We created the SharePoint List as you offered.

Add a datacard into the Form.

The Items of the Combo Box:
[ThisItem.Delegate,ThisItem.Incumbent]

The DefaultSelectedItems of the Combo Box:
If(Int(DateDiff(Today(),ThisItem.'Start Date',TimeUnit.Days)) <= 0 && Int(DateDiff(Today(),ThisItem.'End Date',TimeUnit.Days)) >= 0,
ThisItem.Delegate,
ThisItem.Incumbent
)

We can see the Combo Box is worked from the screenshot.
If we change the date.

It will still worked as we expect.

Hope this will help you.
Best Regards,
Tina