This has no mention of what happens to the prompt cache, including the "learn more" link.

Knowing Anthropic, it wouldn't surprise me if it will result in a full cache miss/rewrite at fallback, with potentially up to 1M tokens in the context window.

https://platform.claude.com/docs/en/build-with-claude/fallba...

Fable requests that fall back to Opus do not charge the cache miss cost of Opus.