A cost usage spike is ignored by the Anomaly Detection report.
AWS is known for delivering CURs late, often well into the following month. For example, AWS may continue to drop CURs for November 2024 well into December 2024.
The new data added in December 2024 might include cost usage for November 19th, 2024 which causes a spike in that day. In this case the Anomaly detection will not detect this spike since it appeared in the Cost reporting after November 19th.