We need to bring back the word pastiche into common usage. Nay, it should be required that any novel created after the author’s death (with a few rare exceptions) should be automatically labeled as a Pastiche.
For example, that “new” Poirot “continuation” novel is not actually a continuation or addition to Agatha Christie’s works. It’s a pastiche. A faithful imitation. Perhaps a very good imitation (I would not know, I have not read them). And yes, it is officially authorized by Christie’s estate.
But it is not Agatha Christie’s work and should not be sold as such. Official or not, it is not penned by her hand, it is not a half-finished novel she left behind, and they should not try to confuse her fans by putting her name so large on the covers.
(The rare exceptions are things like the final Wheel of Time novels or the Tolkien Legendarium. Where the novel/collection is made by someone close to or chosen by the original author to complete a story that was left unfinished. Brandon Sanderson was chosen by Robert Jordan’s widow to write the end of the story based on Jordan’s notes, outlines, and drafts. He finished the story and now the Wheel of Time is complete. He will not write anything further in that world. Christopher Tolkien, already his father’s editor, assembled the books of the Legendarium from his father’s extensive notes and near-complete works, filling in gaps where necessary but not adding anything like new characters or stories or world building.)
(Those are not pastiches. Those are just finishing the work.)