This has been verified on a local implementation and ESRI demo sites. It has also been documented on GitHub repo.
Request sent to GeometryServer probably needs the geodesic parameter set per API doc:
Thanks to Robert Scheitlin, GISP for his suggestion here: https://community.esri.com/message/655101-re-buffer-not-accurate?commentID=655101#comment-655156
For anyone hoping to overcome this and have geometries in a geographic coordinate system, modify the code below. I have not tested this in production but seems to work in test.
roughly line 1270 js\library\widgets\siteLocator\siteLocator.js
params.geometries = [this.featureGeometry[this.workflowCount]];
params.unit = GeometryService[this.unitValues[this.workflowCount]];
params.geodesic = true;