BeforeSave calculations don't fire when collection item is deleted

I have the following calculation.  It returns the total number of days associated with a schedulable object (_DayOff):

import System
static def GetAttributeValue(_DayOff):  totalDayCount = 0.0  if _DayOff.Appointments.Count> 0:    for instance in _DayOff.Appointments:      localTimespan = instance.EndDatetime - instance.StartDatetime      localDayCount = 0.0      localDayCount = localTimespan.Days + localTimespan.Hours / 24.0 + localTimespan.Minutes / 24.0 / 60.0      totalDayCount += localDayCount  Value = 0.0  Value = Math.Round(totalDayCount, 2)  return Value

I am using the following dependencies:

Appointments/StartDatetime
Appointments
Appointments/Deleted
Appointments/EndDatetime

(I added #2 and #3 for testing but they didn't help)

The calc is running perfectly when I add an appointment and when I change the start or end times within an existing appointment.  But it doesn't seem to be firing at all when I delete an appointment.

Syntax checker comes out fine, and Test Calculation always returns the correct value.

I even tried adding a condition to see whether instance.Deleted != null but that didn't help me either. 

The above behavior is reproducible across all calculations based on this Appointments collection.  Am I missing a dependency?