Is the default/current value set to the number that it always changes back to after being cleared?
Try recreating the view but set the default value to 0 (if you are looking to have this be null, there would need to be additional calculations to be done. To return null for when the [parameter] = 0.) this will make the parameter reset to 0 after time the value is cleared.
Above will show 0 when the value is cleared as the current vlaue is what the parameter is reset to when the parameter value is cleared.
Hope this helps!