Been away from the forum for a couple of days, just found this post.
I knew something was off, but didn't really think it through. I should have figured this out, so thanks.
And I just checked one of my favorite filters, Precision Contrast, and it has the same problem. The Shadow control seems to affect shadows the most, but there is definite adjustment to the mid-tones, and slight-but-noticeable adjustment to the highlights. And similar problems with the Highlight slider.
I like the contrast sliders, but wasn't using the shadow & highlight adjusters as I didn't like the results. Now I know why.
I'm using Studio as a plug-in with Affinity Photo/Windows, using Precision Contrast and HSL Color Tuning, instead of Topaz Clarity.
Has this been submitted as a support "Request"?