I have just updated the printer's firmware (from a 2015 to a 2020 one). Now it prints in duplex mode with no complaints. So, it was almost certainly a printer bug rather than a CUPS bug.
-- -Sanjoy
- Bug#1071595: cups: contortions needed to use duplex printin... Sanjoy Mahajan
- Bug#1071595: it was a printer firmware problem Sanjoy Mahajan
