Why account assignment is not deleted when push "Order as Direct Material"?

Hi Gurus,

I have a problem with shopping carts (SC) of stock items:

1.- I add 5 items.

2.- I push "Order as Direct Material" button for 3 items only.

3.- I push "Hold" button to set SC with held status.

4.- I modify the SC and push "Order as Direct Material" button for 2 items missing.

5.- I order the SC.

One of the effects that it has the "Order as Direct Material" button is that it deletes the account assignment because the items will use to create a PO stock.

But for the last 2 items (number 4.-) the account assignment is not deleted.

This is causing that the PO's have error: "Invalid tax code".

Somebody can say to me why is not deleting the account assignment for items previously hold?

My systems are:

SRM 4.0 (SRM_SERVER 500) level 8

R/3 4.7 (SAP_APPL 470) level 24