Hi-Res FLAC files play back at about half-speed (or slower), with half-frequency pitch. This is true in MC21 (my previous version) and now in MC24. Testing so far has involved playing lo-res FLAC files of a song on MC, VLC, and Audacity (no problem in any of those players in lo-res), then playing hi-res versions of the same song in all three players. The other two players handle the file with no problem, while MC has the above issue. MC can also (obviously) handle mp3s of the same song. The second counter takes about 2 seconds to cover 1 second (so it's 2 seconds before the counter flips over from 0:24 to 0:25, for example) in MC. No errors seem to be posted.
I tried to send examples as attachments, but they are too large and of the wrong file type for this forum. The example I would send is Pat Metheny's "Sueno Con Mexico", which has a FLAC version taking 119.7 MB. One can argue about whether that resolution is necessary, but it certainly exists, and it can be played without distortion on other (less audiophile?) platforms.