Not missing anything, as far as I'm aware FLAC can only support the year in the YEAR tag with format YYYY.
My script is not able to recognise this so you will have to copy one of the files in the script directory SEE HERE FOR LOCATION and rename it with FLAC at the end or to your choosing. Then open the script in preferably notepad++ as it's easier to read and edit the last line removing the # like so:
# YEAR OPTIONS
#regexpreplace "(releaseDate":")(\d\d\d\d)-(\d\d)-(\d\d)" "$1$2-$4-$3" # YEAR in YYYY-DD-MM format
#regexpreplace "(releaseDate":")(\d\d\d\d)-(\d\d)-(\d\d)" "$1$4-$3-$2" # YEAR in DD-MM-YYYY format
#regexpreplace "(releaseDate":")(\d\d\d\d)-(\d\d)-(\d\d)" "$1$2" # YEAR in YYYY format
To:
# YEAR OPTIONS
#regexpreplace "(releaseDate":")(\d\d\d\d)-(\d\d)-(\d\d)" "$1$2-$4-$3" # YEAR in YYYY-DD-MM format
#regexpreplace "(releaseDate":")(\d\d\d\d)-(\d\d)-(\d\d)" "$1$4-$3-$2" # YEAR in DD-MM-YYYY format
regexpreplace "(releaseDate":")(\d\d\d\d)-(\d\d)-(\d\d)" "$1$2" # YEAR in YYYY format