While running a test of 20 back-to-back identical prompts, with ~1450 token system prompt prefix, I am consistently getting > 90% cache hit rate when using gpt-5.4-mini. Simply switching the model to gpt-5.4-nano and running the same test always returns a 0% hit rate even after multiple runs.
I am not aware of any difference between the prompt caching capabilities of these models so I do not understand why switching to 5.4-nano would result in a 0% cache hit rate.