One reason for missing keys can be due to how tags are sourced for Flex Reports.

Tags within Flex Reports are currently only sourced from within the CUR.  This means that each tag key you wish to be present within Flex Reports will need to activated as cost allocation tags within AWS as mentioned in the Help Center article linked below.
 
https://docs.vmware.com/en/VMware-Aria-Cost/SaaS/using-and-managing-vmware-aria-cost/GUID-managing-perspectives.html#levels-of-support-for-aws-services-11

Please review your activated tags as described in the AWS article below and reach out to support if a key is missing and is activated.

https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/cost-alloc-tags.html