For Azure, FlexReports can only show Tags associated with a Resource and not a Resource Group.
This can be found in Azure documentation here: Understand your detailed usage and charges - Microsoft Cost Management
However, there is a built-in Policy that can be enabled for the Resources to inherit the Resource Group (Or Subscription) Tags referenced here: Policy definitions for tagging resources - Azure Resource Manager
Additional information regarding manually inheriting a Tag on Resource Group to Resource can be found here: Tag resources, resource groups, and subscriptions for logical organization - Azure Resource Manager