[VBRP]: // Billing Document: Item Data Load [VBELN] as [Billing Document_VBELN.VBRK].....
The left join will merge the data with the [VBRK] table. There won't be a table [VBRP] created because of the join. And since you're only loading Billing Document_VBELN.VBRK which already exists in [VBRK] you will not see any change in [VBRK].
That does not seem possible to me, based on what you posted.
It can only load the field [Billing Document_VBELN.VBRK] since you only load the field VBELN:
SQL Select VBELN from VBRP
Since you're using a left join the table [VBRP] will not be created and the field [Billing Document_VBELN.VBRK] will be joined (i.e. merged) with [VBRK]. In other words [VBRK] does not change and [VBRP] is not created.