Item Substitution is not working as expected in Oracle Fusion

Summary:

Hi Team,
We have item substitutions and its working fine when item is having no quantity.
But below case:
When Item having stock but no available to transact or reserve, at this stage is order is getting error in scheduled.
Is this expected behaviour?
If available to transact is 0 means, it means it has no stock then item should be substitue right?

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