After reading your question, I decided to learn more about FLAC encoding. With an easy web search I found this interesting article written in 2014:
http://z-issue.com/wp/flac-compression-level-comparison/
I can imagine that the Clip Sport processor may not be up to the challenge of decoding FLAC Level 5 compression ?
Just for curiousity, have you tested the Clip Sport playback on a few files at FLAC Level 0 or Level 3 ?
(Suggested by the author of this Flac compression comparison article.)
