If you don't specify a custom icon, apps use the add-ons icon. See https://addons-dev.allizom.org/en-US/apps/app/test-webapp-3/ We need a default icon for webapps.
Apps shouldn't have a default icon. We should require the developer to upload one or choose from our set.
It sounds like this is done, but -> ashort to verify that icon is a required field, then close it up. Thanks.
If the manifest doesn't specify an icon and you don't pick one, you currently get the puzzle-piece icon.