Please note there are 2 similar sounding roles:
 

1. Billing Account Reader is an MCA role: Billing account reader Read-only view of everything on billing account https://learn.microsoft.com/en-us/azure/cost-management-billing/manage/understand-mca-roles
2. Billing Reader: read-only access to the subscription billing information https://learn.microsoft.com/en-us/azure/cost-management-billing/manage/manage-billing-access