Flac is good. But mine’s all ALAC. Which means they work across all my devices. FLAC doesn’t work on Apple devices without massive fannying around - and the user experience of the FLAC apps aren’t always great.
I'm seeing the same behavior as domchocolate. I listen to most my music on an iPod Classic so converting to FLAC isn't really a great option. What's weird is that when the track initially loads, it shows the artist/track/album for a moment before it changes to the number.m4a format.