Still, I would suggest the following formula that might be faster to get the count of issues with a Complete date after the specified date. This would subtract from all issues with the complete date the ones that have a complete date before the specified day.
NonZero([Measures].[Issues with due date] -
Sum(
PreviousPeriods(
[Time].[Day].DateMember(DateParse("2019-12-21"))
), [Measures].[Issues with due date]))
)