Correct, it only filters on whatever stations are currently shown.
And as you found (along with others, see Bring back the filter funnel please), only the main, top right, magnifying glass works.
Searching for hires will show flac streams 96k and over (there are only 8)
Searching for flac does (almost) what you think. Unfortunately, Roon only shows the first 50, and there are actually over 100. There doesn’t seem to be a way of getting Roon to display a useful subset.