The way Roon Radio works now, is that, by default, it is looking both inside and outside your library - however, chances are high that it will discover new music for you outside your library. You can restrict it to only look inside your own library - look at the 3 dots menu at the top right of the Queue screen.
The only caveat is that when looking just in your own library, Roon uses the same algorithm as was in version 1.5 for Roon Radio, instead of the improved algorithm used to swim through the entire universe of known music (i.e. your library, and the entire contents of the streaming services that you are subscribed to).