When you make such fundamental errors in your prompt, it's hardly surprising that the language model replies in a somewhat confused manner.
And second, I think the AI is actually getting mixed up because it is being too literal. The number of letters in the word 'prime' is '5' not 'prime', so the answer is no. It then gets further mixed up and says it is not a prime number because it has already gone down the path of the answer being no.
The prompt that generates confusing results for ChatGPT is, as far as I can tell, a valid question and in correct English.
The problem is that human language is very imprecise and requires too much context to determine the correct meanings of words. If the whole corpus of training data did not inherently have this ambiguity, training an AI would be much simpler and I think such simple ambiguities would be eliminated. The author did not specify which definition of "prime" in the prompt. By clearing stating "prime number" there is no ambiguity, and ChatGPT gives the correct answer.