Hi @Jeremy Wolfgank ,
You have to find the name of column "Doc Type" mappped to managed property in SharePoint search schema. It will take hours after adding the column to the library to wait for the column to be crawled and mapped to a managed property. I would suggest you create a site column named "Doc Type" and add it to your library. Or if you create a column directly in the library, you have to index it or map it manually from crawled property to managed property.
As per my test, I created a choice type site column named "DocType"(no blank between "Doc" and "Type") and added it to a library, I found it was mapped to a managed property named "DocTypeOWSCHCS" after several hours.
I used the KQL text: DocTypeOWSCHCS: "Procedure"
and then I got what I want in the highlighted content web part.
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