Why is my EBS volume unmounted from my EC2 instance at a certain moment?

The event where the EBS volume is detached from the EC2 instance will be captured in CloudTrail logs.

This knowledge document, although it focuses on describing attach/detach history of a single EBS volume, could be useful https://repost.aws/knowledge-center/list-attachments-history-ebs-volume

To help with troubleshooting you could create a metric filter and CloudWatch alarm which will notify you when a DetachVolume event occurs https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudwatch-alarms-for-cloudtrail.html