Okay I have a solution, and think I understand what is going on here… and why it worked for a period of time and stopped. Basically, Logitech’s software is rubbish. It swallows the media keys, and only passes them on to particular programs, and do not provide a way to configure this. There are numerous solutions on the web for their old software, not for the latest one: https://superuser.com/questions/338108/how-do-i-get-my-logitech-media-buttons-to-control-spotify/888413
The interim solution? uninstall logioptions. Now media keys work as I would expect. So this is only partially a roon specific issue.
This unfortunately means I lose various other mouse/keyboard options, but media keys are pretty useful…