Duplex printing not enabled in okular

Hi.

I have a hp color laserjet pro m252dw duplex capable printer.
I can use it as duplex printing, for instance with libreoffice or other software, but okular does not let me print double sided, it is greyed in it.

I can create a document in libreoffice, and print it double sided. If I create a pdf from libreoffice for the same document, open it with okular and try to print double sided the options are greyed.

I have installed kpdf (from kde3) and it lets me use doble sided printing

What is happening to okular?

best regards