Symptoms
A customer has account statement day set to the 1st day of month in PBA, their subscription has billing period type set to Monthly on Statement Cycle Date. This means that subscription is billed monthly on the date of statement cycle which is the 1st day of month.
However, suddenly PBA generated billing order not on the 1st day of month, it (billing order) was generated let's say on the 5th day of month.
Cause
Generation of billing order on the 1st day of month was blocked by non-completed renewal order.
E.g. if grace period in Service Terms assigned to a subscription is set to 10 days, it means that customer has 10 days to pay renewal order after their subscription expired. If customer paid renewal order for their subscription only on the 5th day after subscription expired PBA will be able to generate billing order for the subscription on the 5th day only as well. All the time when subscription renewal order was not paid (processed) PBA could not generate billing order. That is why billing order was created not on the statement cycle day defined in the customer account.