From Header changed by Outlook Rule?

USER1  has Send on Behalf permissions for USER2

When USER1 sends an email on behalf you can see that reflected in the FROM header. 

Strange thing is when this SENT email is processed by an outlook(client based) rule - Copied from SENT, based on Subject, to an Outlook folder - the Send on Behalf is removed and it only shows FROM:USER2

Is this normal behavior? What is actually going on?

Outlook 2010 SP1, Win7X64

Thanks,

SJ