>> ... How do we ask HP to fix this in a driver update ...
I've no idea - most of the support routes seem to be geared more towards hardware issues, and then only if the device is still in warranty (unless you give them a credit card number).
>> ...there is no setting in the Printer menus on the machine itself to disable Duplex ...
I don't have access to M401 models, but my LaserJet Pro 400 Colour MFP M475dn (a similar generation of printer to the M401) has a Duplex setting (On | Off) on the Paper Setup page on the System tab of the Embedded Web Server interface.