Apparently I'm not the first to figure this out but I've seen no mention in our community....
Flac v 1.3.3 (latest). 24bit files will fail FFP with error "format of file xxx is not supported". At first I thought it was the newest Traders Little Helper but I put the new flac in an old version of TLH and got the same error. I then put flac 1.3.2 in the newest TLH and it worked and verified the files. I duplicated it on 2 machines and with many different 24bit file sets. Some my masters and some others. Files seem to encode fine and will test ok but won't pass FFP verification. Only happens on 24bit files.
Below are the same files with newest TLH. First with Flac 1.3.3 then with 1.3.2