Subinventory not automatically bring out together with Item selected?

Summary:

Subinventory not automatically bring out together with Item selected?

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Hi Experts,

Please refer to below screenshot, that:

(1) When PO line (with Inventory type) is brought over from a PR, Subinventory field is having the correct value of "LSFO" automatically assigned.

(2) However, when the PO line #2 is manually added in PO with the same Item "LSFO" chosen just like PO line #1, Subinventory field in Distributions tab won't have any value assigned.
Even after clicking on "Save", there's no Subinventory information bring out.

May I ask why there's a difference on the system behavior?