Billing schedule rows are not deleted when a contract sub-line is terminated -- why?

My user has terminated a sub-line in a service contract.  Normally when this happens, all rows past the current date are deleted from the sub-line's billing schedule.  However, in this case no rows were deleted from the billing schedule at all.

I noticed an unusual aspect of this billing schedule: 

-- There areseveral years of monthly records with $0 as the amount.

-- The last month's record is for $40.

Is this relevant to the question of why no rows in the billing schedule were deleted? 

Thanks for your ideas,

Jennie