The Qt5 based print dialog for e.g. okular, konsole etc. does not use the
default options set for a printer, and also does not remember previously set
options. This concerns options such as Duplex Printing, which is always set to
None even when the printer default (as seen in the Properties -> Advanced tab
or via CUPS) is set to duplex.
The net result is that the user for each print job needs to go to Options ->
Options -> Long Side in order to do duplex printing.

Note that the problem is specific to Qt5 based applications, e.g. firefox and
chrome do remember the settings.

This is a fixed upstream in
https://codereview.qt-project.org/#/c/226881/
See also
https://bugs.kde.org/show_bug.cgi?id=333844
It is a regression from https://bugzilla.suse.com/show_bug.cgi?id=552218 and
https://bugzilla.suse.com/show_bug.cgi?id=645839
As far as I remember, in Leap 42.3 this problem did not exist.