Flac files are only indexed with casesensitive.
How to reproduce :
Assume file in flac format with title "The Flac Song".
Assume file in mp3 format with title "The Mp3 Song".
1. Search for "song" in android sonos app.
"The Flac Song" is not shown in results. "The Mp3 Song" appears in results.
2. Search for "The Flac" in app.
Now "The Flac Song" does appear in results. Album art, artist, title, everything looks ok.
Case is important, as well as the starting point of the search string.
3. Search for "song" again.
Now both mp3 and flac titles appear in results!
The next day the problem returns, because of re-indexing perhaps.
What is going on here?!?