Is it with some special configuration that you are getting this behaviour? Like when the source of the dropdown is report/datpage/clipbaoird page,(or in all you are getting this), and also what is the action you have configured on the dropdown.
Tried the similar use case with Page List as source to the drop down field and calling Refresh section on change event. Able to see the selected value in the clipboard without using post value also, tested in Pega 8.1.3 environment. I feel it would be better to share your use case screen shots along with the configurations and the output results of your actions to understand the special scenario in your use case.