Compatibility with 'invoice-date'?

There is a View ( I think it comes from Commerce Reports) that displays the monthly sales of a business. It works very well, but it is based on the dates the orders were created. We however need such a View based on the dates the invoices for these orders were created. The Commerce Billy module therefore created the 'Invoice-date'-field.

Now if I change that view by replacing the original Created Date field by the Invoiced Date field something strange happens when I check the Views Data Format SQL box.

Before that I have a list of individual orders, each on its own line, like:

-------------------
Order............Invoice date
1..................april 2014
1..................april 2014
1..................may 2014
------------------------------

202 lines in total.

So far there is nothing wrong. The number of orders should be counted per month though, but they are not because I havent switched the Format SQL option yet.
When I do that I get a page like

--------------------------
Order................ Invoice Date
196 (no month is mentioned here)
1...................... April 2014
1.......................January 2014
1...................... July 2014
2...................... March 2014
1...................... September 2014
----------------------------
Total number of orders is still 202, but the oresentation is a total mess. Only some of the months are mentioned and they contain in reality a lot more orders then the 1 that is displayed now

I have been experimenting for many hours now, but I can only conclude that Views Data Format SQL doesnt know how to handle that date field. It may be that Commerce Billy created a specific format, I cannot judge that, but so far it only happens after I switch on the VDFSql switch.

I really hope you have the time to look into this.
Thanks in advance