PCS have confirmed there is a bug which prevents custom buttons from working correctly if they have to call one of the standard print buttons.
Scenario:
I needed to set a global variable for the Excel button before printing so my button was coded as follows

gb_Excel = true
SetFocus(BTN_Excel) 
SendKey("{LEFTBTN,1,1}", BTN_Excel) 

When the button is clicked it activates BTN_Rectangle instead.
Work Around
Don't use an additional button. Instead activate the WM_LBUTTONDOWN process on the standard Excel button, any code in that is processed before the CLICK code

Cheers

David