Ionic v4 + Ionic Native: cordova-plugin-photo-library always returns undefined

I’m trying to get Ionic Native Photo Library to work in a blank v4 project, but I’ve been unsuccessful.
see: https://ionicframework.com/docs/native/photo-library/

Here’s what I have in the HomePage:

  scan_PhotoLibrary():Promise<Observable<LibraryItem[]>> {
    const self = this;
    return self.photoLibrary.requestAuthorization()
    .then( ()=>{
        const photolib = self.photoLibrary.getLibrary();
        if (!photolib){
           // photolib is ALWAYS undefined
          console.warn("IonicNative PhotoLibrary: not available with this Device")
        }
        photolib.subscribe({
          next: library => {
            library.forEach(function (libraryItem) {
              console.log(libraryItem.id);          // ID of the photo
              console.log(libraryItem.photoURL);    // Cross-platform access to photo
              console.log(libraryItem.thumbnailURL);// Cross-platform access to thumbnail
              console.log(libraryItem.fileName);
              console.log(libraryItem.width);
              console.log(libraryItem.height);
              console.log(libraryItem.creationDate);
              console.log(libraryItem.latitude);
              console.log(libraryItem.longitude);
              console.log(libraryItem.albumIds);    // array of ids of appropriate AlbumItem, only of includeAlbumsData was used
            });
          },
          error: err => { console.log('could not get photos'); },
          complete: () => { console.log('done getting photos'); }
        });
        return Promise.resolve(photolib);
    })
    .catch( (err)=>{
      console.log(err);
      return Promise.reject(err);
    });      
  }
}

if I step through the Ionic Native code on the ios simulator, I see it returns void when it should be some kind of Observable:

    PhotoLibrary.prototype.getLibrary = function (options) {
        var _this = this;
        return (function () {
            if (checkAvailability(_this) === true) {
                return;   // <= shouldn't this return `Observable`?
            }
        })();
    };

Any suggestions?

It works if you use the cordova plugin directly, so the problem seems to be in Ionic Native

see: https://github.com/ionic-team/ionic-native/issues/2785