When printing jobs with an odd number of pages from MacOS 10.11, with Xerox driver version 4.20.5, the last page is left out. If I print to PDF file, then use the machine's web interface to submit the PDF file, then it prints all pages. So this seems to be a driver bug.
I can't replicate that since we don't have mac 10.11 to test. (Test labs are current and back 2 for Operating Systems generally, so for a Mac, we have 10.12-10.14)
Call Xerox and report it to see if they can fix it.