FLAG_ACTIVITY_NEW_TASK doesn't open previous activity but only on fresh apk install

Question

I am developing an android library which uses FCM to receive a notification in the background. Upon tapping the notification, the user should be redirected to the last open activity - not the launch activity. I use the flag FLAG_ACTIVITY_NEW_TASK to accomplish this, and the app works as expected in two cases: 1. if I build the app directly over USB in android studio, and 2. If I download and install an .apk of the app, kill the app, and then restart it.

However, if I install the application as an .apk, I get the following behavior:

1. The app opens on the launch activity A.

2. I navigate to activity B and put the app in the background

3. I receive a notification and tap on it. The app opens on activity A instead of activity B as expected.

I don't understand the difference between installing the application as an apk and installing it over usb. Is there some difference in the activity that makes FLAG_ACTIVITY_NEW_TASK treat it like a separate activity? Here's my code:

    Intent packageIntent = applicationContext.getPackageManager().getLaunchIntentForPackage(packageName);
    String mainClassName = packageIntent.getComponent().getClassName();
    Intent intent = new Intent(applicationContext, NotificationTapService.class);

(Note that the reason I'm setting up an intermediate intent that launches a service to reopen the last activity is because the service also calls some unrelated analytics functionality)

    PendingIntent pendingIntent = PendingIntent.getService(applicationContext, requestCode, intent, PendingIntent.FLAG_ONE_SHOT);
    NotificationCompat.Builder notificationBuilder =
    new NotificationCompat.Builder(applicationContext, applicationContext.getString(R.string.channel_id))
        .setSmallIcon(notificationResourceId)
        .setContentTitle(messageTitle)
        .setContentText(messageBody)
        .setStyle(new NotificationCompat.BigTextStyle()
            .bigText(messageBody))
        .setSound(defaultSoundUri)
        .setVibrate(new long[] {1000, 1000, 1000, 1000, 1000})
        .setLights(Color.BLUE, 3000, 3000)
        .setContentIntent(pendingIntent)
        .setAutoCancel(true)
        .setPriority(NotificationCompat.PRIORITY_MAX);
    NotificationManager notificationManager = (NotificationManager) applicationContext.getSystemService(applicationContext.NOTIFICATION_SERVICE);
    notificationManager.notify(messageId, notificationBuilder.build());

(setting up the intent to reopen the activity at the top of the back stack) (from NotificationTapService)

    Intent activityIntent = new Intent(applicationContext, Class.forName(intent.getExtras().getString(CLASS_NAME)));
    activityIntent.setAction(Intent.ACTION_MAIN);
    activityIntent.addCategory(Intent.CATEGORY_LAUNCHER);
    activityIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    startActivity(activityIntent);