The Cambridge Grammar of the English Language, page 122, reads
He was dying is an implicature because of the possibility of cancellation, as in When I last saw him he was dying, but now you would hardly know he had been ill; other speakers might insist on he seemed to be dying or the like, and for them subsequent reaching of the terminal point has the strength of an entailment
Why does the verb seem changes the implicature of die into an entailment?