User-defined tags must be explicitly activated as cost allocation tags in the AWS Billing and Cost Management console before they can be used in Cost Explorer. Simply applying tags to resources is not sufficient.

Once activated, cost allocation tags can take up to 24 hours to appear in Cost Explorer, but they will not appear at all if activation is not performed. The 30-day delay applies only to historical reporting after activation, not to visibility itself.

Cost and Usage Reports and Budgets are not prerequisites for Cost Explorer filtering.

Therefore, the issue occurs because the tags were not activated for cost allocation.