The search functionality seems to be working fine. Apparently, we where searching the wrong the whole time. Search for SO only allows numbers so if I want to search for a sales order, I should just input numbers on the search field.
Another question though, is the search box allowing searching for purchase orders also. On further investigation on the controller.js. SAP has initialized the search for PO but it didn't finish coding it. So when we search for purchase order e.g. "test purchase" it doesn't yield any result.
Does the standard sales order tracking search functionality provide buttons or drop down to tell user that he is searching for sales order or purchase order?