The post object field, like the relationship field, searches post content and post titles. It can only find text in these two fields. This likely means that orders do not use these fields.
I do not know anything about how WC stores orders so I cannot give you any specific information. What you will need to do is figure out how the orders are stored and locate the data you want to search. Then you will need to modify the search query “WHERE” statement to include this data in the search in conjunction with acf/fields/post_object/query.
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