Keywords in Photos 6 on Big Sur - Discrepancy between the Search Results in Smart Albums and the Filter for Hierarchical Keywords

I like to use hierarchical keywords to tag photos, if the photos are showing objects of hierarchical categories, for example the keyword Animal_Bird_Sparrow for the category "Animal", the subcategory "Bird" and the category "Sparrow".

I noticed after the upgrade to Big Sur, that there is discrepancy in the search results between searching with a filter or searching with smart albums. The "Showing Only" filter seems to be behaving similar to the text search and respond to sub words in the full keyword (Changes to Searching for Text in Photos 5 Catalina)

My test album has five photos of birds, three robins with the keyword "Animal_Bird_Robin" and two Blue Tits with the keyword "Animal_Bird_BlueTit".

• When I search with a smart album for "Animal" with the rule "Keyword is Animal", the smart album is empty, because none to the photos has the keyword "Animal". The same goes for the keyword "Bird". Thus has always been this way - the keyword search is searching for the full keyword.
• But when I filter the album with the filter "Showing only Animal", all five bird photos will be found, because the keyword contains the substring "Animal" as a partial word. Also "Showing only Bird" will respond to both the keywords "Animal_Bird_Robin" and "Animal_Bird_BlueTit".

I am not sure, if it is a bug or a new feature. But it is very convenient to filter for parts of hierarchical keywords. We do not have to assign the keywords "Animal", "Bird", "Robin" separately, the Filter can select the parts of the combined keyword between the underscore characters. Only, we have now to be careful, to define the keywords in a way, that we are not unintentionally using the same word in unrelated keywords.

The filter result for "Animal". As you can see in the Keyword Browser, none of the photos has the keyword "Animal" assigned, only the combined keywords, prefixed by "Animal".

The same test for "Bird":