This seems expected. You have searched for the text you entered within the Discourse topic you were currently looking at. That is not the Ask AI feature but the classic search limited to the ID of the current topic. It’s not about limiting search to topics within the forum.
And even with Ask AI, you only search within the forum, not elsewhere about content in the forum. That’s what you would do with an ai-bot. Ask-ai is about better search within the community.