Make sure in the List Lookup B definition you’ve defined Dependent Lookup = List Lookup A and then for all the Lookup Keys you specify one or more Dependent Values from List Lookup A,
Then on the table associated with that application, on the Filtered Lookups tab, add an entry in the Dependent List Lookups table where Controlling Field = List Lookup A and Dependent Field = List Lookup B.
After you’ve done that (and a possible app regeneration) I think you should see the expected list lookup dependency.
(FYI I’d be interested to see what the Nextworld chatbot agent Ed would come up with if asked this type of question)