Why do gpt-5.1 and gpt-5.4-mini behave so differently in production chatbot use cases?

Yeah, in experimental phases for new features on Production I do similar. Start with large model, fine tune the code and prompts until I’m satisfied, then later step down the model via settings and see if I can retain acceptable behaviour until I find unacceptable cases, if any, then step back up. You could do this in some kind of staging environment too if your risk tolerance is less, of course.