The problem is that such formatting was adopted by Sarah Ferguson -- now addressed as "Sarah, Duchess of York" -- after she divorced Prince Andrew in 1996. And Harry's mother was known as Diana, Princess of Wales after her divorce from Prince Charles.
According to the palace, the titles of Duke and Duchess of Sussex are not subject to change, but updated guidance for how the names should be formatted is being prepared.
Buckingham Palace said its previous guidance on the royal couple's title was erroneous, and it is now conferring with the Garter King of Arms -- who specializes in matters of titles and protocol -- to issue updated guidance.
Meghan was given the title Her Royal Highness The Duchess of Sussex when she married Prince Harry in May 2018.
Harry's full title was His Royal Highness The Duke of Sussex, Earl of Dumbarton and Baron Kilkeel.