We've released version 4.1.7 of cordova-plugin-ionic. 4.1.7 addresses issues affecting use of the 'background' update method on iOS.
Unfortunately the recent 1.2.0 release of cordova-plugin-ionic-webview is incompatible with Deploy. In the near term it will be necessary force the following versions to use deploy on iOS devices. Verify that the ~ and ^ characters are not present in the version spec in config.xml and package.json to ensure the correct version is used.
We will continue to work on a permanent solution for deploy on iOS. If you'd like to be notified when an update is available please open a ticket with support or follow our Product Updates section to receive email updates.