ItemPurchaseOrderService returning Orders converted to Bills

The ItemPurchaseOrderService seems to be returning Orders that have converted to Bills.

I don't think it should.

Example:

I create Order No. 00000010 in AccountRight.

I change it to a Bill
It now has a new order number of 00000016 (I don't know if this is a good idea, but anyway.. )

When viewing it in Purchases Register, it appears under Open Bills as 00000016
and no longer appears as an Order with no. 00000010

This is all as you would expect.

However, if I retrieve all orders using the ItemPurchaseOrderService API
Order No. 00000010 still appears as an Order with Status set to Open (not ConvertedToBill)
and Order No. 00000016 also appears as an open bill.

Once it is converted to a Bill, I presume that it should not be returned by the ItemPurchaseOrderService.
I can't see any field that I could filter on to exclude Orders that have been converted to Bills.

Any suggestions wecome.