Refund for a resource downgrade is calculated incorrectly if a customer has paid for this resource in advance.
The issue is caused by an internal software issue #PBA-82568 ["Duration of resource downgrade item is incorrect in case resource was billed for two periods"]. It affects only subscriptions with "Charge Before Subscription Period" or "Charge Before Billing Period" billing models where a resource is downgraded after it was prepaid for the next period. Because of this issue, refund calculation does not take into account period that was payed in Billing or Renewal order but is not used yet.
A customer's balance can be adjusted (for example, by issuing a Credit Memo document).
Contact your Technical Account Manager or Pooled Technical Associate Team at firstname.lastname@example.org to clarify the status of the #PBA-82568.