Try using the WHITE slider to remove your blown-out highlights instead of the Highlights slider.
The HL slider works on the 1/4 tone area of the histogram rather than on the end-points.
Understood, but not really concerned about the blown highlights, just the fact that moving the highlight control to such a negative value would have increased the clipped area - also, this highlight clipped display is really not working - I have an area on a image where the RGB values are 170, 165, 180 and it shows as clipped - surely this is not correct behavior as nowhere near a clipped value - but the clue here is where a lighter area is next to a much darker area.
So something is not right with the highlight clipping display - change the color temp and the clipping goes away. Very prevalent on high ISO images - so maybe high ISO noise is coming into play also
Can you post an example image that exhibits the behavior?
We were discussing this in another thread several months ago
So, under certain conditions and when some camera profiles are used (both Nikon and Canon), it can happen that trying to reduce clipped area with sliders actually increases that area. It's caused by the profile as made by the factory (Nikon, Canon) and isn't ACR issue. I think I also posted a graphical representation of one profile showing why it happens. If not, I can post it
The fact is - sRGB / Adobe RGB gamut is smaller than gamut of raw image, so some colors are clipped even if they are not clipped in raw color space, sometimes in slightly unexpected way. However, if Adobe standard profile is used, clipping is performed according to ACR workflow which also has some drawbacks, which we also discussed in another thread on raw image from Olympus camera, several months ago
Showing area with RGB 170,165,180 as clipped is unexpected - can you post the sample ?