My mistake.
I was making an assumption you were going back 24 months not to jan.
{INVOICE_DATE} in yeartodate
or
{INVOICE_DATE} in lastyearytd
Make a formula field as
totext(month({INVOICE_DATE}),'00') + ' ' + totext(year({INVOICE_DATE}),0,'')
Group on that for your order to work.
You can make an inverted one to display as year month