Default:

Expand the Paste Mask by 10mil:

Set the Paste Mask Expansion to **-**10mil:

Paste Mask resets to the default value of 0mil expansion BUT the indicated value of the expansion attribute remains at 10mil:

            
                        
            
                                    
              
                Chrome
                  73.0.3683.103
                Windows
                  10
                                EasyEDA
                  5.9.42