When a feature layer polygon fill is set to "No Color" in a webmap, the onClick event no longer fires inside the features. It will continue to work when a border path is clicked on, and the popup continues to work just fine, but the event does not fire when the user clicks inside a feature.
No idea why this is, but it seems like a bug. Workaround has been to set the fill to 100% transparent.
See fiddle for example (make sure you click both inside a feature and right on the edge path of a feature to see different behavior)