Here are steps about how to use Department column to filter search results in the highlighted content web part. You could check whether you have appropriate settings.
1.Create a Company Policy content type based on the Documents content type.
2.Insert a site column named Department into the Company Policy content type.
3.Find the managed property name of Department column.
4.In the KQL query, set the formula:olDepartmentOWSTEXT: "Information Technology".
5.Result:
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