Comment 6 for bug 443132

I have checked and yes it is doing as you described.
Unfortunately the provided solution doesn't feel good as one field is now combined for 2 purposes.
It would be better to have a own field at picking and invoice that store the customer reference at creation time of both documents. this require some more work on reports and the need for a new table field.
For our customer we build last week a customization that does this. I will request the developer to submit his patch here.
Then we can decide what is best for stable and for trunk version.