Cannot set default printer to remain in documents

I am not sure whether this question got posted already or not. Something went wrong, so here it is again.

I now have an Epson XP-750 printer-scanner. I have set it as my default printer in System Preferences. When I create a document whose paper size is one of the standard ones catered for by the XP-750 or open a previously saved one in one of those sizes, or I open a document from one of the templates provided by Pages, the printer shown in Page Setup is always the XP-750. But if I create a document in a custom size (such as a 9 cm x 14 cm correspondence card), Page Setup shows 'Any Printer' until I re-set it to 'XP-750', and even if I save the document or save it as a template and then re-open it, the printer is always shown as 'Any Printer'.

Is there any way of making the XP-750 'stick' as the printer shown in Page Setup even in these custom documents? Or is this an Epson problem and not an Apple one?