Thank you, Harmut.
I was copying parts from a catalog to a "temporary spec", then exporting to catalog builder. This property was not being carried over into the excel file, although the column exists in the spreadsheet. Subsequently, when I used the "Build Catalog" command to bring these parts back in to my catalog, the "Part Category" property was blank.
I've since switched my workflow, and now copy parts to a "temporary catalog". This seems to keep the "Part Category" data intact.
Even going back into the spreadsheet and filling in the "PartCategory" column in excel, then rebuilding the part to the catalog, the filter still ignores it and acts as thought that property is empty.