Hi SRK,
I am following the same steps that you have mentioned!
I am doing VF01, click on the selection list, the open quantity field is editable, I change the quantity and select the line and click on copy and do F8 - until this step this is fine but when this is executed, in the billed quantity field, it is displaying the original quantity and not like in the screen shot that you have posted.
Thanks!
Best Regards