Date String "Gone!"

That’s why I ask if the first record was deleted. This probably removed the formula when the record was deleted.
This is why I always have two header rows in my sheets. Row 1 is used for Column Heading Name, and Row 2 is used for formulas. What you need to do is insert an empty row right below the headings, then re-add the formula to cell F2 of the new empty row. You may get an error because you still have the formula you already re-added and is now in cell F3. Just clear out that cell and it will repopulate the Date String all the way down.
It’s also possible that you still have Date String as an editable field in your app. If its still there, you need to make sure to remove it.