My imported book used Gemini for "Extend with AI" although I write with another engine. Why?

Before version 1.9.23, an imported book had no AI engine of its own, so everything the AI did for it went through the free chain, which starts with Gemini. From version 1.9.23, a newly imported book gets the engine you chose last, when it is still connected. For a book you imported earlier, open its page, then Book details, and choose the engine in "AI engine for this book": Extend, regenerations, Quality and the editor's AI tools then use it.