The ZoomScale property in the Search widget does not work for polygon layers.
Bug ID Number
October 8, 2015
December 1, 2020
Will Not Be Addressed
The development team has considered the issue or request and concluded it will not be addressed. The issue's Additional Information section may contain further explanation.
This is designed this way. The zoomScale is only applicable if there isn't an extent on the result. Points, generally don't have this.