For your interest, I created an image in Affinity Photo that is a simple gradient…
The histogram shows a series of small “teeth”, which, to me, indicates that the gradient has “notches” of varying luminance.
Then I adjusted the four Selective Tonality sliders and created a Tone Curve to increase the contrast…
The result is…
… where you can start to see banding appearing, due to the Selective Tonality.
Whereas, if I only use the Tone Curve, with a gentle lead in at both ends…
The banding is a lot less obvious…
All this to say that I rarely use Selective Tonality because of this problem, which can, effectively, “sharpen” the transitions in bokeh.