That reminds me of something I have been wondering about: On the one hand, most royal watchers (and yes, the Countess of Wessex) argue that the Wessex and Sussex children are legally or at least legally entitled to be Princess/Prince unless and until the 1917 Letters Patent are explicitly replaced by new Letters Patent, regardless of any other expression of the monarch's will - but on the other hand, no one seems to question the validity of the 2012 Letters Patent creating the Cambridge children Princess/Prince even though it contravened the 1917 Letters Patent (which prohibits new creations of Princesses/Princes), and nobody appears to question that William is truly the Prince of Wales even though the monarch has not issued letters patent to formalize it.
For the avoidance of doubt, I do not take sides on the argument about letters patent versus other expressions of the monarch's will concerning royal titles. However, in general, the deference to Letters Patent seems to be argued selectively, and I cannot help but wonder if there is some unconscious bias being projected because William being Prince of Wales and the Cambridge children being Princess/Prince from birth feel "correct" to most royal watchers, whereas male-line grandchildren being denied the status of Princess/Prince is more controversial.
I don't see any connection between the cases you state. In the Wessex and Sussex case the Letters Patent of 1917 hold because there has been nothing to contradict them. In the case of the Cambridge children the Letters Patent of 2012 does contradict those of 1917 and they are newer so they take precedence. There is nothing unusual here. 1917 holds unless supplanted with newer (as in 2012).