Ok, I've found the issue: I don't know why, but in the printer default settings
the duplex tool option was set to disabled (it must have been changed during the upgrade
process somehow, because in F29 the duplex option was working).
For the notes, the relevant option is in System settings -> printers -> Configure
-> printer options -> installable options
Or in CUPS webpages you have to look in the printer page under 'Set default
options'
Thanks all
Mattia