What version of ACR? What version of Photoshop? Hosted by Photoshop or Bridge?
You've not provided much info...
This isn’t how ACR sliders act for me. What sort of input systems are you using during the time: mouse, trackball, touchpad, tablet, touch-screen, keyboard, something else? It could be a driver issue or maybe a handicapped-mode that’s somehow enabled where something is getting translated as a double click unexpectedly. I saw this because usually what resets a slider back to zero is double-clicking on the slider-handle, or perhaps the slider-title-word.
If there is an ACR bug and not just a weird driver problem that only affects your system, it should be repeatable by others.
Can you provide description of every mouse move, drag, click, double-click, etc, and any keys pressed including shift and alt and ctrl, starting with your moving the highlights slider up to 20 via some input device, until you move the shadows slider and it resets the highlights back to 0, and include the position/number of the two sliders in your example as well, along with any screen elements that are highlighted at any point in time.
It would help to know whether you’re on a PC or Mac, what version of PS and ACR you’re using so others can attempt to replicate in as close to the same environment as possible.
Did ACR ever work correctly for you? Is this the first time you’ve used PS and ACR or the first time you’ve used it on a particular hardware configuration on this computer? Does it act the same on other computers or is the situation unique to just one?