Invoice updates undoing gift certificate assigned value

Using Soap web services to create and update invoices, including assigning gift certificate values as part of the payment. The header/items and the gift certificate value are set in separate requests (both use upsertList, with the latter setting the giftCertRedemptionList value and the former not setting it at all).

However, after an invoice update (that does header/items) to an invoice that had a gift certificate value assigned, the assigned gift certificate value has now changed, often to a value that is equal to the 'gross amount' of the first line of the invoice, instead of the amount originally set.