I have no idea if this is new with 5.9... may have pre-existed and I just did not detect.
With regard to FLAC music I have on a USB drive:
* Despite the fact that all of the tracks are tagged accurately (verified on my mac), when I browse the device in media screen, it must be using its own sources to look everything up and determine artist and album. Some things it finds right, some things it cannot find at all. What's weird is it finds album, track name, art, etc for Turn It Over by Tony Williams Lifetime (totally obscure), yet cannot place a Rolling Stones song from Exile on Main St.
* As a result, if I browse by song, everything shows up (tho often without album or artist), but if I browse by artist or album, a bunch of content not there
* It is definitely looking up its own album art. Which it sometimes finds, sometimes not. Reason I can tell is that the art that shows up is often different version than the art I have associated (tagged) to the track.
Yikes, this is a mess! Anyone else experiencing this? I think I've seen similar in posts, but since this thread is 90% release anxiety posts vs. feature discussion it's so hard to wade through and find.