On Field Update Set do not fire Browser Script BusComp_PreSetFieldValue in BC

Hi,

I have set up a On Field Update Set user property in BC. When field A gets updated, will update field B in Action BC.

There are also some other logic we need to perform after the value change on field B, so there are some script made on Action BC as browser script -  function BusComp_PreSetFieldValue.

Then found out then field B is updated manually in UI, the broswer script fired normally. However, if the field is updated by User Property, the browser script seems can not be triggered.

Please help to explain this action in siebel, is there any perticular reason doing such?