This isn’t true, you’ve got it mixed up with William’s daughter.William's children would not have been princes until last Thursday according to the old George V rules.
The Queen stepped in upon Prince George's birth to change the old rules and give him the title of prince.
The "row" was why she did this for one grandson and not the other.
You pinning it all on a Royal convention that the Queen tore up 5 years previously isn't a very strong argument.
William’s son was always going to be a Prince, as he’s the heir to the throne.
Charlotte wasn’t going to be, because she’s female.
The Queen changed the convention to stop discrimination against female Royals.
She didn’t change it for Harry’s children because it would have grown the Royal Family again, which is something they’ve tried to do the opposite for the last 100 years, for obvious reasons.