I have a Genre: Mixable, which I use to tag tracks which I like to hear in a random mix.
I have the Mixable genre results set saved as a bookmark.
When I recall this bookmark and click Shuffle, Roon complains that “Nothing similar found: Limiting Roon Radio to library”.
This list has hundreds of popular artists in it.
Roon has no trouble seeding Roon Radio with these artists when I use them directly.
It should be happy to take any such playlist as a seed rather than fixating on the Genre:Mixable alone…