I installed Roon just over a week ago. I’m also trialling Tidal.
I experienced the same issue when I first connected Tidal to Roon. It fixed itself the next day.
This morning Roon Radio was working on my phone as an audio output. Then about an hour later I used a different phone to control my 851N network streamer. The issue returned and Roon Radio was limited to my library.
I suspected that Tidal does not allow more than one active playback connection, so I went back to my first phone. On this phone, Roon playback was paused on a Tidal track since there is no stop. I then played a track from my library to get it off Tidal. Then I went back to my 851N and Roon Radio was not limited to my library anymore.
Hope my experience helps with your issue.