approved orders sometimes set to "Pending Billing"

We are encountering a strange problem with some sales orders in Netsuite.  Normally when an unapproved order is approved by a manager, its status changes to "Pending Fulfillment."  That's great, the order shows up in the pick ticket printing queue, etc.  However, in some cases the order's status becomes "Pending Billing."  It still lists the items on the order as committed from inventory, but it doesn't appear in the queue of pick tickets to be printed.  If we don't notice this problem, the order won't be shipped out.

If we edit this bad order and set its status back to "Pending Approval" and approve it again, the status correctly goes to "Pending Fulfillment."  It's just the first time that sometimes doesn't work.  Has anyone else encountered this?  I'm on hold waiting to talk to support, so I'll post a case # when I have it.