Selective Tones are not only about tone curve but also local contrast. Positive ‘Blacks’ add a lot of microcontrast, aimed at getting dark details more visible. In some specific cases it’s better to use Tone Curve, combined perhaps with fine-contrast Shadows slider. See https://forum.dxo.com/t/visible-posterization-in-pl8-in-bokeh-areas/40346 for another possible problem and some technical background.
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