Index: src/wp-admin/js/customize-controls.js
===================================================================
--- src/wp-admin/js/customize-controls.js	(revision 33720)
+++ src/wp-admin/js/customize-controls.js	(working copy)
@@ -2107,6 +2107,15 @@
 		},
 
 		/**
+		 * Prevent users from being able to skip cropping. Site Icons must always be cropped.
+		 *
+		 * @since 4.3.1
+		 */
+		mustBeCropped: function() {
+			return true;
+		},
+
+		/**
 		 * After an image is selected in the media modal, switch to the cropper
 		 * state if the image isn't the right size.
 		 */