Authors Failing at Tone

I feel like a lot of authors want to have their cake and eat it when it comes to tones and themes, and it just doesn't work most the time. Like there will be a light fluffy story and the it turns out there was a rape backstory, or there's a dark psychological but with wacky moments. If its a good author, they might be able to get away with it, but if it's something under 12 chapters, it fails of most of the time and one casts a shadow over the other. And I think shows a lack of confidence to be unable to just stick to one idea and commit to it.