Hi @Allen_Chen and thanks for reaching out!
My hunch is that this is directly linked to the recent changes we made to sections in order to implement Switching from Boards to List View (and vice versa).
Sections used to be just like tasks in our backend which explains why they were returned in Advanced Search results, but they have now their own model which could explain why they aren’t showing up in the Advanced Search anymore.
Let me check this with our Development Team and I’ll be in touch with more info!