I'm facing this same issue.
The thing is that when sorting with 'year' instead of year(yyyy)', the sorting is still wrong, since it doesn't understand the days, months and years, it just sorts alphabetically. If the dates are 04/04/1999 and 01/07/2014, MusicBee thinks that 01/07/2014 goes first because it starts with 01 and the other 04.