Android Studio app restarts upon notification click, no matter what I try

Question

I am honestly starting to believe the issue reported in the title is an issue with the new Android API required to publish on the store. I have tried setting android:launchMode="singleTask" in the manifest, and below is my code to build and show the notification. I have tweaked and tweaked but whenever the notification is pressed, the app restarts.

//other info: Android Studio: 2021.3 Emulated: Pixel 4 XL API 30 SDK: Android 11 Also tested on physical device: true Cache invalidated: true

//CODE

private void showNotification(int notificationID, String title, String message, String channel_id, String channelName, String channelDescription, int notifIcon)
    {
        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
            int importance = NotificationManager.IMPORTANCE_HIGH;
            NotificationChannel mChannel = new NotificationChannel(channel_id, channelName, importance);
            mChannel.setDescription(channelDescription);
            mChannel.setLightColor(Color.RED);
            mChannel.enableVibration(true);
            mChannel.enableLights(true);
            mChannel.setVibrationPattern(new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400});
            mChannel.setShowBadge(false);
            mChannel.setLightColor(Color.RED);
            notificationManager.createNotificationChannel(mChannel);
        }

        NotificationCompat.Builder builder = new NotificationCompat.Builder(this, channel_id).setSmallIcon(notifIcon).setContentTitle(title).setContentText(message);

        Intent resultIntent = new Intent(this, MainActivity.class);
        resultIntent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);
        TaskStackBuilder stackBuilder = TaskStackBuilder.create(this);
        stackBuilder.addParentStack(MainActivity.class);
        stackBuilder.addNextIntent(resultIntent);
        PendingIntent resultPendingIntent = stackBuilder.getPendingIntent(0, PendingIntent.FLAG_UPDATE_CURRENT);
        builder.setContentIntent(resultPendingIntent);
        builder.setOngoing(true);//s that it can't just be swiped away!
        //
        notificationManager.notify(notificationID, builder.build());
    }