Test Banner Ads don't load on Android 11.0 - Flutter

I have a flutter app setup to run google ads using the google_mobile_ads package. All packages are the most recent. I configured it for iOS & Android and the test ads are loading on iOS as expected but not on Android 11.0. This is only an issue on Android 11.0 - API R. When run on Android 9.0 - API 28 the test ads load.

My configuration: AndroidManifest.xml

<meta-data
  android:name="com.google.android.gms.ads.APPLICATION_ID"
  android:value="ca-app-pub-MY_APP_ID"/>

initState()

setState(() {
 _banner = BannerAd(
   adUnitId: 'ca-app-pub-3940256099942544/6300978111',
   size: AdSize.fullBanner,
   request: AdRequest(),
   listener: bannerListener,
 )..load();
});

where bannerListener is:

  final BannerAdListener bannerListener = BannerAdListener(
    onAdLoaded: (Ad ad) => print('Ad loaded.'),
    onAdFailedToLoad: (Ad ad, LoadAdError error) {
      ad.dispose();
      print('Ad failed to load: $error');
    },
    onAdOpened: (Ad ad) => print('Ad opened.'),
    onAdClosed: (Ad ad) => print('Ad closed.'),
  );

When I run on Android I get the following error:

AdWidget requires Ad.load to be called before AdWidget is inserted into the tree

If I change the code up to split the ..load() to a different line I then get this error:

I/Ads     (11598): Ad failed to load : 0
I/flutter (11598): Ad failed to load: LoadAdError(code: 0, domain: com.google.android.gms.ads, message: Internal error., responseInfo: ResponseInfo(responseId: null, mediationAdapterClassName: , adapterResponses: []))

I'm showing the banner with this code:

if (_banner == null)
 SizedBox(height: 20)
else
 Container(
   height: 50,
   child: AdWidget(ad: _banner!),
 )

What am I doing wrong that the test Ads won't load on Android but iOS has no issue?