ionic cordova platform add fails with local plugins

**Description:**
Hi there!
This happens since about 2 days. When i add a platform like ios with 'ionic cordova platform add ios', it fails, because i have local plugins added to my config.xml.
When i use cordova platform add ios instead, it works.
Am i supposed to only use cordova platform add?

**Steps to Reproduce:**
ionic start myApp tabs
(i copied a plugin to ProjectDir/local-plugins/PluginDir)
ionic cordova plugin add local-plugins/PluginDir
ionic cordova platform add ios -> fails
rm -rf platforms
rm -rf plugins
cordova platform add ios -> success

**Output:**
[ERROR] An error occurred while running subprocess cordova.
--verbose gives me no more infos

**My `ionic info`:**
Ionic:

ionic (Ionic CLI) : 4.1.2 (/usr/local/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 : ios 4.5.5
Cordova Plugins : cordova-plugin-ionic-keyboard 2.1.2, cordova-plugin-ionic-webview 2.1.4, (and 4 other plugins)

System:

Android SDK Tools : 26.1.1 (/Users/schoko/Library/Android/sdk)
ios-deploy : 2.0.0
ios-sim : 6.0.0
NodeJS : v8.12.0 (/usr/local/bin/node)
npm : 6.4.1
OS : macOS High Sierra
Xcode : Xcode 9.4.1 Build version 9F2000

**Other Information:**
I tried to roll back ionic, app-scripts ... but no change.

Reproduce the failure from the listed ionic start, local plugin add, and ionic cordova platform add ios commands, then compare it with cordova platform add ios. Start at the Ionic CLI command path that invokes Cordova and determine the cause of the local-plugin failure. Done means the Ionic command succeeds with the same local plugin setup, with a regression test if the project has coverage for this path.

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