Let me clarify: I have not seen ChatGPT make any grammatical mistakes in its use of Japanese in continuous Japanese-only text. Like you, I have seen it make mistakes in explanations of grammar, both about Japanese and about English, as well as in its translations between the two languages. Its explicit knowledge of grammar seems much worse than its productive grammatical ability.

If you spot any productive Japanese mistakes by ChatGPT, let me know. The About page of my website, which is linked from my profile page, has my e-mail address. I see from your site that we share an interest in the Japanese language. I would be happy to hear from you.