With Ghostscript 9.18 we were able to print A5 duplex on Xerox and HP printers. After GS 9.25 was installed (security update?) the same A5 duplex print fails on same Xerox and HP printers: simplex is printed. No problems are encountered with A4 duplex printing.
I reported this issue as Ghostscript Bug 700232. Kudos for the developer who fixed the bug . The fix will be shipped in next Ghostscript release, planned March 2019.