I wonder if "X is not Y - its' Z" LLM shibboleth is just an artifact of "is not" being a third most common bigram starting with is, just after "is a" and "is the" [0]. It doesn't follow as simply as it does with markov chains, but maybe this is where the tendency originated, and later was trained and RLHFed into the shape that kind of makes sense instead of getting eliminated.

[0] https://books.google.com/ngrams/graph?content=is+*