AdMob ad doesn't show until I reopen the app

Question

I'm trying to put an AdMob banner into my app but half the time it doesn't show any content. The AdListener even receives an call to onAdLoaded and I never get any error.
I noticed that if the ad isn't showing and I put the app in the background somehow the ad starts to show. For example if I:

1. close and reopen the app
2. start an fullscreen interstitial ad
3. start the in-app purchase overlay

All these actions trigger an update that somehow redraws/remeasures the views.
I tried many different things to simulate this in onAdLoaded.

Is it possible that AdMob can't find a matching ad even if I don't get any errors? According to the website the match rate is 98%. The number of impressions is about a third of the requests.

I found more posts with similar issues (all from ~7 years ago) but none of their answers worked in my case:
Admob not showing up until phone is locked
AdMob won't show the banner until refresh or sign in to google plus
Android AdMob: addView doesn't show ad until return to activity
Admob ads appear only after first refresh

I tried to get this to work for over two days. Please take a look at my code, I annotated it with explainations:

MainActivity:

private AdView adView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    binding = ActivityMainBinding.inflate(getLayoutInflater());
    setContentView(binding.getRoot());
    if(privacyPolicyAcceptedAndHasNoPremium()){ //at first i check the shared preferences
        binding.adPlaceholder.setVisibility(View.VISIBLE); //the adPlaceholder starts as GONE when i set it to VISIBLE the rest of the content slides up
        List<String> testDevices = Arrays.asList(AdRequest.DEVICE_ID_EMULATOR, "...");
        RequestConfiguration requestConfiguration = new RequestConfiguration.Builder().setTestDeviceIds(testDevices).build();
        MobileAds.setRequestConfiguration(requestConfiguration);

        MobileAds.initialize(MainActivity.this, initializationStatus -> {});

        adView = new AdView(this); //i create the adView programmatically but using the xml view element causes the same problem
        adView.setAdSize(AdSize.LARGE_BANNER);
        adView.setAdUnitId("...");
        adView.setAdListener(new AdListener() {
            @Override
            public void onAdLoaded() { // this gets called so i think the ad is successfully downloaded from the google server. i put all kinds of code in here to force the adView to show
                //setContentView(binding.getRoot());
                //binding.adPlaceholder.addView(new View(MainActivity.this));
                //binding.adPlaceholder.forceLayout();
                //binding.constraintLayout.requestLayout();
                //adView.invalidate();
                //adView.bringToFront();
                //adView.setVisibility(AdView.GONE);
                //adView.setVisibility(AdView.VISIBLE);
                //adView.setBackgroundColor(Color.TRANSPARENT);
            }
        }
        AdRequest.Builder adRequestBuilder = new AdRequest.Builder();
        binding.adPlaceholder.addView(adView); //i add the adView into the empty LinearLayout. i tried to add it directly to binding.constraintLayout too.
        adView.loadAd(adRequestBuilder.build());
}

activity_main.xml:

<androidx.constraintlayout.widget.ConstraintLayout
    android:id="@+id/constraintLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">

    <!-- this is the empty container into which i load the ads. it sits at the bottom of the screen and is not visible before an ad is showing -->
    <LinearLayout
        android:id="@+id/adPlaceholder"
        android:layout_width="match_parent"
        android:layout_height="100dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        android:orientation="vertical"
        android:visibility="gone" />

    <!-- this MotionLayout contains the rest of the app (except the toolbar). it slides up when an ad is showing -->
    <androidx.constraintlayout.motion.widget.MotionLayout
        android:id="@+id/motionLayout"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layoutDescription="@xml/activity_main_scene"
        app:layout_constraintBottom_toTopOf="@+id/adPlaceholder"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:context=".MainActivity">
    ...
    </androidx.constraintlayout.motion.widget.MotionLayout>

</androidx.constraintlayout.widget.ConstraintLayout>

Manifest:

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<application ...>
    <meta-data
        android:name="com.google.android.gms.ads.APPLICATION_ID"
        android:value="..." />
    ...
</application>

Edit:

Answer 1

I finally found a way that works for me.

I'm just setting the window background color to the color that it already has. This is really fast and doesn't recreate the activity.

However I don't know why this works and why only this worked for me. I guess it updates the views in a similar way to the many other solutions from the other questions.

adView.setAdListener(new AdListener() {
    @Override
    public void onAdLoaded() {
        super.onAdLoaded();
        getWindow().setBackgroundDrawableResource(R.color.background);
    }
}