You may get a clue from the content of the history. Is there a new history step with the exposure reset or was that step deleted?
I'm not that familiar with acr. Don't use it often. Wasn't aware there was a history panel in acr.
Something new I discovered this morning which may be a work around until I get an answer. If I hit the auto button I can then move the sliders below exposure without the exposure slider moving on its own. Then, as a last adjustment I can move the exposure slider from where it is if necessary.
Still, if I hit auto, move some sliders, then move exposure, if i change any of the other sliders, click in a box to change a number on any other adjustment, exposure goes back to 0 or the nearest full stop on its own.
Weird adobe photoshop/acr behavior I've never had to deal with cause I was using LR for raw work.