I think r&b is a genre built on performance and the conviction of it. So whether it's about being sexy or being heartbroken or being loved-up or being angry, the best r&b will take those emotions to their extreme - sexier than sex itself, more tears than the ocean's water, the greatest love any human could feel beyond comprehension, the fury of a thousand demons, etc. I don't believe that authenticity is necessarily the only or best way to conjure an emotional response in a listener, but I believe that in the framework of r&b the idea is to convince the listener - hence, performance. Even when I know it's not real - Beyonce isn't really in the limo, Usher isn't really about to make love to someone in the disabled toilet at the club, etc - it's about believing in the situation, real or otherwise, projecting yourself into the fantasy of that performance.
So when I hear someone make an obvious sidestep into territory where they sound uncomfortable and ill-fitting, it's distracting. When the singer doesn't sound at ease in their performance, when they're in a space that they don't naturally fit, it draws attention to the artificiality of the performance, and diminishes the effect of conviction.
I think about when Nelly Furtardo did that album with Timbaland and Danja and got away with it because she fully committed to the role and everything needed to make that work. I think about when Little Mix tried to pass themselves off as being into reggaeton because everyone else was doing it for a summer and it just sounded like a graft. Those are the extreme examples, in reality it's the kind of thing that marks the mediocre from the good from the exceptional.