CloudHealth does not enforce Account Affinity to the application of RIs through Partner Billing.
For example, if account A has an RI and an instance and account B just has an instance, CloudHealth does not prioritize applying that RI towards the instance in account A (all of this is under the assumption the RIs and instances match type/os/region). This may lead to potential costs not lining up exactly as expected as RI benefits may not fall exactly within account lines.
When troubleshooting any issues with specific account costs not matching bills, this is one item to keep in mind during your investigation.