Digital Light & Color

I have an image which shows "clipped pixels" in clouds when I try to brighten the image even though the clouds are not even close to white. In fact, they are somewhat grey. Other areas of the photo which show clipping when brightening make sense in that they are close to white or have bright reflections.
Any idea what is going on here? The original image was created by a lab scanning a negative.

Pixels show as clipped if any of the channels is clipped.

If you brighten using HSL, only pure white has a lightness of 100%. If you brighten using HSV, if any channel is 100%, the value is 100%.