Symptoms
There are billing orders which were generated not on the customer's statement day even though all the configuration is done currently - subscription are supposed to be billed on the account's statement day. The affected accounts have statement day set to 31.
Cause
The described issue is caused by the bug with id #PBA-79910 ("Two billing orders for subscriptions that belong to customers with statement day 31").
Resolution
Contact TAM or PTA to get information about the status of the bug #PBA-79910.