You have asked Legacy to 'apply' double dating, so it does.
But as far as I can see, Legacy always takes a year off, rather than adding one on! It is something that has puzzled me for a while.
Like you, I don't always notice a date should be double dated, but Legacy always does, so it would seem we have 2 choices - enter the later year when we do realise, or rewrite it when we don't!!