I am experiencing the exact same issue with either Dsub or Ultrasonic detailed above by apastuszak (Files that play just fine using folder structure are missing when browsing by ID3 tag) using a fresh install of Madsonic 5.1.5080.
For example, I have the an artist by the name of 10 Years in Madsonic. Using the folder view, 10 years shows directly at the top (alphabetically) & can see and play all the tracks. If I switch to ID3 tags, 10 Years disappears. I can search for the artist and play songs that way, but they just do not show up in the view. Switching back to folder view, and voila the artist appears again.
I also validated that that all of my mp3 files are tagged properly (ID3 v2.3) & also tried using both Dsub and Ultrasonic to compare. Both clients experience the identical issue when switching over to ID3 tags so I'm guessing this is a server bug. Refreshing the library, deleting/starting over for the library, has no effect.
I was going out of my mind here trying to figure this out.
Anyone else experiencing this anomaly?