Printing manager automatically duplex-prints on short-edges

Dear all,

this is a very odd issue that it had never occurred to me before.

When I am in a hurry I tend to just select the PDF files needed and hit Cmd-P for the system to marvellously handle everything as it should. Duplex printing is normally done as it should (long-edge binding) and I have never encountered any issue before.

Tonight I hit Cmd-P on a bunch of PDFs and they all got printed with the binding set on short-edge.

I looked for options in the Preferences of the Printer but nothing, nor in Preview.

Do you know what may cause this and how to set it so that it automatically prints double-sided documents with the binding set on the long-edge?

FYI, I am on a 2016 15" MacBook Pro with macOS 10.13.3 (including the last supplemental update of a few days ago).

Thanks for your help.