1.4.26 - Saving existing Sales order removes first line item

[freelock]

I thought we had this resolved, but we're still seeing the issue. To trigger it, we followed these steps:

1. Create sales order for a customer with a qty of 25.
2. Open this sales order, enter 10 in the "Ship" qty field, and click Sales Invoice. So far, so good, the invoice shows 10 to ship, original sales order is updated to qty 15.
3. Reopen the sales order, now showing qty 15.
4. Change the qty to 10, and click update.
5. Save, and confirm you want to overwrite the existing sales order.
6. Re-open the sales order and the first line item is gone.