This tool is definitely faulty after recent updates. I have created images with different tonal ranges (by just using Levels) and this is the result on my machine.
On an image in which the tonal range goes from 0 - 255 the tool works correctly.
If the tonal range does not go all the way to 255 the tool does not work correctly at all and actually moves the tonal range up to 255. The Shadow and Highlight slider both have this effect as soon as the Amount is increased from 0% to 1%. This should not happen and did not used to happen.
This happens in Photoshop CC 2018 and some previous versions.