User-generated notes without an associated concept ID are displayed in the "Detached" state. This includes, but is not limited to, user-generated notes that were associated with a since-edited portion of text, and user-generated notes that are associated with a concept that is hidden during flashcard quizzing (you can hit "More Details" and/or "Show related topic details" to reveal the additional context). Firecracker does not display detached notes in the "Answer Notes" because those notes are not associated with any concept within the answer.
Users can re-associate these notes with a new concept by hitting the blue "Attach" button, then dragging their cursor over a word or phrase and hitting "Reattach note." Here is a quick video demonstration of the Notes functionality, timestamped to start at the detached notes section: https://www.youtube.com/watch?v=oBQnQIiqVhY&t=78