AndroidManifest.xml does not register BroadcastReceiver but app will register programmatically?

I can't seem to find a recent tutorial on this and the ones from 2017 and earlier don't appear to work. But if I do the broadcast receiver registration in Java it will work fine. What am I doing wrong here?

If I was to add the below to MainActivity::onCreate it works fine whether or not the manifest has the <receiver> entry:

registerReceiver(
                new DemoReceiver(),
                new IntentFilter("com.example.receiverdemo.CUSTOM_INTENT")
        );

Basic example:

MainActivity.java:

package com.example.receiverdemo;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        findViewById(R.id.TestButton)
                .setOnClickListener(view ->
                    sendBroadcast(new Intent("com.example.receiverdemo.CUSTOM_INTENT"))
                );
    }
}

DemoReceiver.java

package com.example.receiverdemo;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.util.Log;
import android.widget.Toast;

public class DemoReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        Log.i("FOO", "com.example.receiverdemo.DemoReceiver::onReceive called.");
        Toast.makeText(context, "Intent Received!", Toast.LENGTH_SHORT).show();
    }
}

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="com.example.receiverdemo">

    <application
        android:allowBackup="true"
        android:dataExtractionRules="@xml/data_extraction_rules"
        android:fullBackupContent="@xml/backup_rules"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.ReceiverDemo"
        tools:targetApi="31">
        <activity
            android:name=".MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <receiver
            android:name="com.example.receiverdemo.DemoReceiver"
            android:exported="true">
            <intent-filter>
                <action android:name="com.example.receiverdemo.CUSTOM_INTENT"/>
            </intent-filter>
        </receiver>
    </application>

</manifest>

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/TestButton"
        android:layout_width="wrap_content"
        android:layout_height="48dp"
        android:layout_gravity="center"
        android:text="TEST BROADCAST RECEIVER" />

</FrameLayout>

There is a restriction above Android O.

Note: If your app targets API level 26 or higher, you cannot use the manifest to declare a receiver for implicit broadcasts (broadcasts that do not target your app specifically), except for a few implicit broadcasts that are exempted from that restriction. In most cases, you can use scheduled jobs instead.

https://developer.android.com/guide/components/broadcasts