Override the plant on a returns order with reference to billing document

I am trying to over-ride the plant value to be 1000 for all lines, for all returns orders that we have, no matter what plant was on the original referenced document.

We usually create returns orders (RE2) with reference to either an order, or invoice.

I placed code to check for AUART = RE2 , etc   and then override with vbap-werks = 1000  in USEREXIT MV45AFZZ -- move to vbap.

From a standard order reference, it works fine, but when we create the return with reference to an invoice, it hits the code that I placed MV45AFzz, but the value gets set back (in later code) to the original plant(s).

Not 100% sure why it reverts back for invoices and not standard orders, but I've been debugging through lots of downstream code in the order include.

Can anyone help with whether I will need to place more/further code to keep the invoice-referenced plant override to 1000 (and where that code should be) and , or if it is a config setting, etc.?

Thanks

Jeremy H.