Project Costing DFF - Unable to clear value in user defined Unit DFF when project is blank

We are using the Project costing DFF on the Expense Report Line. We have fields to enter the Project Number, Task Number, Expenditure Org and Unit field. The unit field is using USER_DEF_ATTRIBUTE1.

The requirement we have is that if user removes the selected project number, then the Expenditure Org and Unit field also should get cleared out. We observe that as soon as project number is made blank and user tabs out, the expenditure org also becomes blank. But the Unit field retains the value that user had entered/selected from LOV. How do we clear this value as well?