I am also running into this issue on iOS 11 - with “viewport-fit=cover” the app only displays white screen.
If I reload page with safari development tools, then it displays properly the second time.
Perhaps it is worth mentioning that this app was built with earlier version of Ionic 2/3 and I had to add the viewport-fit tag manually.
cli packages: (/usr/local/lib/node_modules)
@ionic/cli-utils : 1.12.0
ionic (Ionic CLI) : 3.12.0
cordova (Cordova CLI) : 7.0.1
@ionic/app-scripts : 2.1.4
Cordova Platforms : ios 4.4.0
Ionic Framework : ionic-angular 3.7.1
ios-deploy : 1.9.2
Node : v6.11.2
npm : 5.3.0
OS : macOS Sierra
Xcode : Xcode 9.0 Build version 9A235