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

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Solidus Version: all

To Reproduce

See attached video

https://github.com/solidusio/solidus/assets/141220/75fc326a-1f97-4a8f-9570-ae2d2f5ead65

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.