mobile billing missing device credit

Hi.  We recently moved out of the xfinity service area, and had both internet and mobile, and had purchased phones via xfinity as well and had monthly device credits (one per line).    We kept the mobile for a few months and paid the extra fee to do so.  on the last day of our billing cycle for this month (which was yesterday), we finally switchedover to another provider at the end of the very day.  However, today the bill came for the last month and our device credits were missing from bill.  I understand that we need to pay for the final payment of our devices without the credit since we are no longer customers.  But I do not understand why it was removed from our last bill when we had a full month of service.  Can anyone help?