BPA Purchase Order Lines WHO column not updated after approval

Summary

BPA Purchase Order Lines "LAST_UPDATE_DATE" Not Updated After Approval / Change of the Line Status from INCOMPLETE to OPEN.

Content

Hi,

BPA Purchase Order Lines "LAST_UPDATE_DATE"  Not Updated After Approval / Change of the Line Status from INCOMPLETE to OPEN. Is this an expected behavior? 

If above is true, are there any specific reason due to which the same approval process updates the header status and the WHO columns but decides to update the line status and leave the WHO columns of the line untouched.