@Viviennemary
I agree she is not Meghan Duchess of Sussex. She is The Duchess of Sussex. The first one would only be used after divorce. Before Diana's divorce she was The Princess of Wales. After the divorce she was Diana Princess of Wales.
It is not because of the divorce that Diana’s name was styled as such but rather it was because of the loss of the title HRH. Exactly the same as Meghan. It is the loss of the title that leads to the name structure.
As Meghan is still married to Harry, that name remains as is. She is Meghan, the Duchess of Sussex. The Firm/Institution does not have it wrong. It’s absolutely in keeping with the precedent set with Diana.
At least on this I agree with the Firm.