Pdfwrite with the attached XPS file generates an XObject (/R15) without a resource dictionary, calling "/R14 Do". R14 is defined in the page resource dictionary. Table 4.45 in pdfref17 says that the resource dictionary for a type 1 form xobject is optional, but strongly recommended. In pdf 1.1 xobjects used the page resources. In pdf 1.2 the xobjects are page independent. Thus it's considered bad form to use the page level resources for xobjects.
*** Bug 693322 has been marked as a duplicate of this bug. ***
This is now fixed, I believe by commit cfeb9f98b896384ac366f4e30f3653a52de7af87