DFF field retains its value even after it is set NULL by COPY()

Hi All,

Using the custom PLL, I am trying to set NULL to one of the DFF fields on HR Assignment screen. For some reason, Form field still keeps the value while the debug message shows NULL for same field. there are debug messages(fnd_message.debug) before the COPY built-in and after, it does show NULL after the COPY built-in but if clicked on DFF window, the value is still there on the field.

This COPY is actually the last in the custom PLL for WHEN-VALIDATE-RECORD event and there is no custom code getting fired at least in WHEN-VALIDATE-RECORD event.