Why are pretax ded not taken for multi-assign EE in different TRUs and insufficient $ in nonprimary?

Summary:

Workers with multiple assignments in different TRUs do not have pretax deductions taken when they have insufficient earnings in the non-primary assignment, even though the primary assignment has plenty of funds to cover the deduction where the benefits are associated. Oracle states this is working as designed and to use the Deduction Register to identify the employees with "Deductions Not Taken". We process payroll biweekly for 20k+ employees so to identify employees after the fact is both burdensome and frustrating to our workers. It is surprising to hear this is "expected behavior". How does your team identify these instances