stock_split_picking: splitting a delivery for a SO with an MTO draft purchase order linked breaks the PO quantity

[gurneyalex]

Issue with stock_split_picking, at least in 17.0

For a product in MTO + buy, if I split the delivery of a sale order before the purchase order is confirmed, the purchased quantity is increased by the split off quantity.

Steps to reproduce the behavior:

1. Create a product in MTO + buy, with a supplier
2. Sell 100 units of the product, confirm the sale order
3. Go to the delivery order, split 30 units off the 100 units delivery
4. check the purchase order: the purchased qty is now 170.

Expected behavior

The purchase order has a quantity of 100

If the PO is confirmed before the split, then everything works fine.