As explained in T168434, the current (default) setting for $wgPropertySuggesterClassifyingPropertyIds is inconsistent with the data that PropertySuggester uses on wikidata.org. $wgPropertySuggesterClassifyingPropertyIds shoudl be set to [ 31, 279 ] to be consistent with the data generated by the PropertySuggester python script.
P279 is the "subclass of" property. This change makes it so that property suggestions depend on which class the present item is a subclass of. This should be a noticeable improvement to the quality of property suggestions on items that represent classes of things.