I have a similar issue.
I have an iif statement:
IIf([plant]="MULTIPLE" And [Discrepancy Code] Like "A*",IIf([tCC_Master].[BU]="Neurovascular","1188","1139"),Nz([plant],IIf([tCC_Master].[BU]="Neurovascular","1188","1139"))) AS DC
and the null Neurovascular records are still returning 1139 in...