[Backend] Backordered items may become on hand when splitting shipment

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Solidus Version:
all

To Reproduce

See attached video

SOLIDUS_BUG.mov

Current behavior
When a shipment item has a mix of back ordered and on hand inventory units, when splitting only part of the item quantity on another shipment, all the inventory units become on hand.

Expected behavior
Inventory unit states are left untouched.