I’m using the core LR keyword functionality.
OK, so I take a new image. Go to the “Keyword list” and add a top level keyword (A1). Include on export and export containing keywords.
Create Keyword Tag inside A1 called B2, Include on export and export containing keywords.
Create Keyword Tag inside B2 called C3, Include on export and export containing keywords.
Check on C3 and add keyword to selected photo. Move up to to “Keywording” tab and check the “will export” dropdown which shows A1, B2 & C3.
Go to Publishing Manager and check the Metadata tab, check ‘Write Keywords as Lightroom Hierarchy’. Publish New photo to WP.
Go to Media Library in WP and only C3 is showing. A1 & B2 are missing.
Hope this helps.