I just checked this behaviour whilst waiting for 2020.44.25 to finish download.
Odd behaviour of the audio source selection and no mention of USB, even though USB at that time was playing. I will check whether the current source playing has an effect on its visibility another time.
The odd behaviour best described in this brief video.
As for USB search, whilst not conclusive, I am pretty sure that non of my searches returned anything from USB. This is indicated by the source seemingly to be a Spotify source and when turning Spotify off, I got a completely different set of search results - just an observation that search was also much faster in this situation.
A small lag in returning some results and same with album art. Difficult to be certain whether I was seeing USB or Spotify media being played. If Spotify, I was pleasantly surprised by some of the breadth of its content and also, it didn't sound too bad even with rain/sleet on the roof which prevented a definite A/B test - I think Spotify was playing but that may just have been a bias.
A final observation. Voice search was pretty good with high level of matching.
I shall repeat with 2020.44.25
For reference, my USB folder structure is below with artists and tracks a level down further than I remembered. Media files are FLAC with full metadata including iirc track based album art - I need to double check that. Thats another one that catches some media players out - album art at folder level not track level.
Music / CD / <artist> / <tracks>
System Volume Information