It was the subscription's billing date, the subscription looks billed (the "Last billing date" is changed), but there is no billing order.
Possibly, the Zero Billing order was generated, which is not visible in BA control panel. It may be zero because of the zero prices configured in the service plan, the setting "Show Zero price" set to "No" in the corresponding subscription period and the absence of the overused billed resources, or the 100% discount/promotion the customer has.
The behaviour described is correct. An order may only be found in the BA database. If you want to see zero billing order in BA control panel, you may set "Show Zero Price" to "Yes" for the recurring fees in the corresponding subscription period in the service plan or in the promotion (if a promotion is a reason of zero prices).