Often when a partner switches a set of customer accounts to a different billing account duplicate billing data can be seen on the partner-customer tenant. This will cause the partner to update the customers billing block and potentially process the entire months billing using the new configuration. If inflated costs are seen for a month (possibly two) and there has been a recent change in billing accounts, this can be the cause.
To verify, view cost reporting in the partner-customer tenant and categorize by billing account. If there is a double-up on billing data due to the issue described, you will see a report similar to that shown in the below screenshot:
To resolve, submit a ticket to Technical Support to have the duplicate costs cleared.