My older book's title is the entire prompt and it prints on the export title page.

Books created before the working-title feature carried the prompt as their title. Since v1.6.4 those books show a notice on their page with a "Suggest short titles" button — click it, pick one of the AI's suggestions (or click the title and type your own), and your exports print a proper title page. BookyAI never changes titles silently; you choose.