PREVIOUSMONTH - Sum vs. Count using ID relationships

Hi,

I am trying to use the previous month function to grab the Sum, Count and CountRows just to try all the options here. Previosuly I was summing aggregates of transactions. Now I have a denormalized table and am trying to count the rows of the table instead to work out the values.

For some reason, I am not getting anything consistenatly returned from the below:

I have two reports that are identical from what I can see, containing the 3 measures below:

Sum Previous = CALCULATE(SUM('Test Data'[Value]) + Sum('Test Data'[Custom]), PREVIOUSMONTH('Test Data'[Date]))

CountRows Previous = CALCULATE(COUNTROWS('Test Data') + COUNTROWS('Test Data'), PREVIOUSMONTH('Dates'[Date]))

Count Previous = CALCULATE(COUNT('Test Data'[Value]) + COUNT('Test Data'[Custom]), PREVIOUSMONTH('Dates'[Date]))

In Report Test the Previous Month function works with Sum. But not with Count or CountRows.

In Report Test 2 the Previous Month function works with Count and CountRows. But not with Sum.

What is going on? It seems like a bug. But I may have missed something.

Further to this, when working with a date table, should my date key always be converable to a date, I have been given a new date data set and the PK is now an ID rather than convertable to date in PowerBI. As seen in Contoso Sales Sample.

Regards,