I've just installed MC28 for the first time and the Playback Range setting for my MKV file worked fine. I then went back to MC27 and the previous problem did not reoccur! I also now have no problem with audio files on MC27. In all cases, I've been using the embedded player via the MC UI without any special zone settings.
My main focus has been on MKV files, where it was immediately obvious that they always started playing from the beginning whatever setting had been used for Playback Range. However, I now realise that my test of a FLAC file was flawed. I was assuming that the player would display the selected start cue, when it actually always shows 0:00 for the start. The volume was turned down, so I didn't hear any variation in the starting point of the music. It's still possible that I had an issue with Playback Range on audio files, but it's no longer present.
I'm struggling to understand how the MC28 installation could have fixed anything in MC27. Can anyone offer a plausible explanation?