Simple cumulative sum issue with live data

So, in case anyone gets here in the future ... I suspected this was a data type issue and this is what I did ...

I exported my data to Excel and made a new report off the Excel file in which, of course, the measures work as expected.

On loading the Excel file, BI transforms my datetime column to datetime in M, like this:

and it looks like a datetime in the canvas

and it builds a date hierarchy off it in the Data pane.

In the live version, it claims to believe it's a (greyed-out) datetime but doesn't create the hierarchy (or prompt you with dot-notation when creating measures off it) ...

CONCLUSION:  in the live version the measures don't work because it's not quite treating that field as a datetime somehow, even though it says it is

EDIT: the incoming data is a datetime from stream analytics - as it's end-to-end MS stuff I'm going to ask them!