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

I would not rely on the scorecards or generic indexes/benchmarks for production use cases. Use a strong model to build yourself a dataset of a few hundred examples for your specific use case and then benchmark your use case (for quality, speed, cost) with a few models and parameters. You’ll be surprised of what you discover.