Cordova-plugin-firebasex under iOS do not rigger callbacks

Firebase Cloud Message notification work quite well under Android.
But in iOS notifications definitely arrive as verified in the Xcode console but callbacks like onMessageReceived() or with getToken() never get triggered.

Anyone with this issue?

Ionic:

Ionic CLI : 6.3.0 (/usr/local/lib/node_modules/@ionic/cli)
Ionic Framework : @ionic/angular 5.0.5
@angular-devkit/build-angular : 0.900.4
@angular-devkit/schematics : 9.0.4
@angular/cli : 9.0.4
@ionic/angular-toolkit : 2.2.0

Cordova:

Cordova CLI : 9.0.0 (cordova-lib@9.0.1)
Cordova Platforms : android 8.1.0, ios 5.1.1
Cordova Plugins : cordova-plugin-ionic-keyboard 2.2.0, cordova-plugin-ionic-webview 4.1.3, (and 10 other plugins)

Utility:

cordova-res (update available: 0.10.0) : 0.9.0
native-run : 0.3.0

System:

Android SDK Tools : 26.1.1 (/Users/fascox/Library/Android/sdk)
ios-deploy : 1.10.0
ios-sim : 8.0.2
NodeJS : v13.8.0 (/usr/local/Cellar/node/13.8.0/bin/node)
npm : 6.14.3
OS : macOS Catalina
Xcode : Xcode 11.3.1 Build version 11C504