Whitney definitely had vocal problems right up until her tragic passing. If you listen to her final performance of "Jesus Loves Me" at one of the Grammy parties on the evening before she passed, she was barely able to sing. These legendary ladies have their moments of vocal rough patches and vocal brilliance after all of these years of singing so hard. The human voice is not meant to endure consistent use and abuse to the extent they had to during their early years of constant touring.
In case you want to see a fairly recent impromptu performance of Mariah singing [[because she STILL can), here you go...
https://twitter.com/kbyrd2/status/81...971585/video/1