Ah, I had to add the Tag Dump group manually, as did many, because our libraries have been around long before the Tag Group functionality was added.
I assume for the "use local copy" function to work, the drive letter of the file on the Server is the same as the drive letter mapped to the same Server location from the Client? Otherwise the library wouldn't work properly. In fact if you had "Fix broken links" set to "Yes" strange things may happen if the drive letter used on each PC was different.
I don't use the "use local copy" function on my Client, and I see raw tags for all audio files (MP3 and FLAC) on the Server from my Client. I guess if you turned that setting off, restarted MC, and the tags appear, you know you have found the problem.
Also, for specific FLAC files where you can't see the raw tags, do they appear on the Server? Perhaps they don't have raw tags in them, if it doesn't happen for all FLAC files.