Strange behaviour with SAMEPERIODLASTYEAR

Hi,

I'm building a Power BI report which is analysing PoS transaction data so have a fact table (Transactions) with every individual transaction including timestamp and sales amount (Transactions[Sales]).

I've added a custom column for the transaction date (Transactions[Date]) which is joined to a calendar table (Calendar) which is generated from MIN(Transactions[Date]) to MAX(Transactions[Date]).

I've created measures for

Total Sales = SUM(Transactions[Sales]
Total Sales Last Year = CALCULATE([Total Sales],SAMEPERIODLASTYEAR('Calendar'[Date]))

The problem occurs when I include the last date in the Calendar table when calculating Total Sales Last Year.

As you can see below, if I select 13 - 14 July 2019 the calculations are correct i.e. the Total Sales Last Year are for the corresponding 2 days in 2018.

Note the the Date slider is Calendar[Date] and the Date column in the table is Transactions[Date].

If I change this to be 14 - 15 July 2019 where 15 July is the last date for any transactions, the Total Sales Last Year is incorrectly calculated for the period 14 - 30 July.

I have found a workaround which involves adding some additional days to the end of my Calendar table but the problem will occur again if these future days are selected in my date filter.

Am I using this function incorrectly or is there a bug?

Thanks,

Michael