Setattribute() not able to update non-null custom field in SR

Summary: I have a custom text field 'Field_A' with value "ABC" in Service Request object. I want to update this value to "DEF" in a Object Function. When I try to update with setAttribute(), the new value does not replace the old value. when I execute the function and check its value through run time messages using println, it shows the updated value. but when I check the same field value in postman through get operation, it shows the old value only.
Setattribute() works only if previous value(before executing function) is null. I even tried setting the value to null before updating to "DEF" in function, but no use.