same problem occured with icons and splash.
better look at the sample icon.png and splash.png given in the boilerplate template. then give your images of exact sizes. worked for me.
Ionic:
ionic (Ionic CLI) : 4.1.1 (~/.nvm/versions/node/v9.8.0/lib/node_modules/ionic)
Ionic Framework : ionic-angular 3.9.2
@ionic/app-scripts : 3.2.0
Cordova:
cordova (Cordova CLI) : 8.0.0
Cordova Platforms : android 7.0.0
Cordova Plugins : cordova-plugin-ionic-keyboard 2.1.2, cordova-plugin-ionic-webview 2.1.0, (and 7 other plugins)
System:
Android SDK Tools : 26.1.1 (~/Android)
NodeJS : v9.8.0 (~/.nvm/versions/node/v9.8.0/bin/node)
npm : 5.8.0
OS : Linux 3.13