I also just learned about this the other day and was likewise shocked. I never heard her in person, but I always considered her to be an uneven singer based on what I did hear. She could be sublime and then problematic. Yet as Charlie notes, based on her performances at Covent Garden she was proclaimed as one of the best Minnie's ever in La Fanciulla del West. In addition the DG recording based on that production not only received the Gramophone Magazine award for Recording of the Year in 1978, but has withstood the test of time as well.