[cups] Lexmark MX310 changing printer format

[Lorenzo Milesi]

I've a weird situation in a Linux-only office. It has been happening for a
while, at least since Ubuntu 18.04 and now they've all been upgraded to
20.04 it's still an issue.

The printer paper is configured to A4 for both manual and main trays. After
some time, and we weren't able to figure out when, the main tray paper
setting turns to Letter.
I double-checked the printers configuration, and they all have A4 as paper
format.

I don't see what could change this setting... Can any PC do it?
thanks