In the Event tab of the Widget properties there is a Refresh option.
Did you explore that? (when you toggle the switch you an try to set that and see if that puts the widget back to its initial state)
Are you using this widget? -> https://marketplace.mendix.com/link/component/116541
I think in your case the data is not clearing on the client side only, have you checked the values during debugging? Is the response attribute empty as expected after your change object activity?
When refreshing this widget it sets the values back to whatever is set as default selected string (under the General tab). Try adding another attribute and set the default selected string in the widget to '’ and then set the refresh boolean to true.