Using Masks (Subject, Background), LRC changing Exposure to zero

Just to be clear, even if you see zero Exposure within the masking panel this does not mean that the general Exposure value for your photo - as set in the Basic panel - is at zero. That slider still remains however you've set it - it works independently of the masking panel.

In fact you can continue to update global settings (in the respective panels lower down the interface) at the same time as overriding parts of the image with masked adjustment. LrC displays the dynamic result, from combining all of that.

To explain what's going on with the masking panel's settings: if you check the box to have those reset, each time you make a new mask it will start out with zeroed local settings regardless of what has happened before (while the whole photo's global settings remain in place). 

The masking panel can retain a default for all its adjustments, which may potentially NOT be zeroed. If you select to place (say) a new Brush mask and begin brushing, default adjustments are initially used. Changing those further, applies only to this particular mask. If you select to place a new Brush mask and immediately change the adjustment sliders before you start brushing, that updates the defaults for these to match. The auto 'reset' checkbox option forces everything back to zeroed again, for the next new mask you will create.

Uncheck the box that says "Reset Sliders Automatically".