They used to, they don't anymore.
The rules were relaxed under the Cameron coalition government as the future Cambidge children (Louis and Charlotte) wouldn't have been automatically entitled to become Prince and PrincAS suhesses but first born George would have been.
The question is whether Harry and Meghan were under the impression that the rules were to be relaxed again- and why they were under that impression. Is it the case they have erroneously jumped to that conclusion, or were they actively [by false representation] or passively misled i. e. by an ommision of communicating the true facts and circumstances and the knock-on effect of Charlie's desire for a smaller monarchy to them.