IMO at that time she was so out of it on drugs, carried away with herself and fame that her voice had morphed into a caricature of itself. Like an older actress who's performances are way over the top. Think Carol Burnett's Norma Desmond and I know I'm dating myself. This is Stevie's voice back then.
https://www.youtube.com/watch?v=eTKhw-v5h2I