Colouring pages are line drawings: clean black outlines on white, with enough enclosed areas to be worth colouring in. Turning a photograph into one means throwing away almost everything the photograph contains and keeping only its edges.
That is why subject choice decides the result before any setting does. A photograph with a clear subject, good separation from its background and strong shapes converts into something recognisable. A busy scene converts into a tangle of short lines that reads as noise.
Portraits are the common disappointment. Faces depend on subtle tonal gradients rather than outlines, so edge detection finds either too little to draw a likeness or too much and turns skin texture into scribble.
Busy scenes, low-contrast images, anything with a cluttered background, and portraits. All of them depend on tone rather than outline, so edge detection either finds too little or turns texture into noise.
Too much detail is being kept. Fine texture such as foliage, fabric weave or skin becomes a mass of short lines. Reduce the detail setting, or use a simpler photograph.
The image is sent to KDP Tools to be processed, because the conversion runs on the server rather than in your browser. Both your original and the generated file are deleted from the server once the job finishes. Deletion happens as part of that job, so do not treat it as a guarantee for material you could not risk being transmitted.