Took a look at PO_MaterialReq_UI.pvc. With Customizer UI Script Events, by creating the Panel OnExit entries for pHeader and pHeaderW, the system expects to find OnExitpHeader and OnExitpHeaderW functions in the UI class. However those functions (methods) don't actually exist in PO_MaterialReq_UI.pvc !! This omission is a a neglected duck situation (ugly duckling??). You should report it to Sage. It's an easy fix.
Even in PO_Return_UI (Rtn of Goods Entry), there are OnExitpHeader and OnExitpHeaderW functions, both of which are doing nothing at all but by just existing it enables Panel Exit UI scripting.