I have not exactly the same problem but it is probably related.
On Ubuntu 18.04.3, i choose Duplex "Long Edge" but it print a single-
sided PDF document.. It was working on Ubuntu 16.04.x.. I tried Ubuntu
19.04 and it works too.
I use Evince for my PDF. If i choose another pdf reader like
Status changed to 'Confirmed' because the bug affects multiple users.
** Changed in: cups (Ubuntu)
Status: New => Confirmed
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https://bugs.launchpad.net/bugs/1782719
Title:
