How to add aggregate site reports in a nested multi-module project to the site of a parent multi-module project with Apache Maven?

Question

Update: I've created a Short, Self Contained, Correct Example (SSCCE) here.

I'm currently refactoring a multi-module Apache Maven Proof-of-Concept project and have an issue regarding reporting plugins or the Maven Site generation.

My structure is as follows (list intent == inheritance):

• Company POM
  • Company Bills of Material (BoM)
• Aggregator POM
  • Code Project Aggregator POM (all source code projects)
    • Library JAR
    • Application JAR
  • Code Coverage Aggregator POM (a non-source code project)

The Company POM uses Convention-over-Configuration and does not define any inherited reports, but only "single" reports, e.g:

<reporting>
  <plugins>
    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-pmd-plugin</artifactId>
        <reportSets>
            <reportSet>
                <reports>
                    <report>pmd</report>
                </reports>
            </reportSet>
        </reportSets>
    </plugin>
  </plugins>
</reporting>

The top-level "Aggregator POM" does not define any reporting plugins. It's main purpose is to aggregate and allow to separate the "source code" and "non-source code" child projects:

<modules>
    <module>pom/code-project-aggregator</module>
    <module>pom/code-coverage-aggregator</module>
</modules>

The "Code Project Aggregator POM" defines the aggregate reports with <inherited>false</inherited> and includes all source code projects as modules:

<modules>
    <module>../../lib/foo</module>
    <module>../../app/bar</module>
</modules>

<reporting>
  <plugins>
      <plugin>
          <groupId>org.apache.maven.plugins</groupId>
          <artifactId>maven-pmd-plugin</artifactId>
          <reportSets>
              <reportSet>
                  <id>aggregate</id>
                  <inherited>false</inherited>
                  <reports>
                      <report>aggregate-pmd</report>
                  </reports>
              </reportSet>
          </reportSets>
      </plugin>
  </plugins>
</reporting>

The problem is, that the generated Maven Site for the top-level "Aggregator POM" does not include the reports if calling mvn verify site-deploy.

The reports are generated if mvn verify site-deploy is called for the "Code Project Aggregator POM". But I want to create the top-level site. I don't understand what is happening here: It seems that the aggregate reports are not bound to site if called on the top level.

Two questions:

1. Can someone explain this behavior?
2. Can someone provide the solution (or guide me into the right direction) that generates and includes the aggregate reports in the child multi-module project if calling mvn verify site-deploy on the top multi-module project?

I've read Building multi-module sites, but couldn't find any mention of issues regarding my approach.

Update #1 2023-06-23: I've looked at the Effective POM of "Code Project Aggregator POM" and the aggregate reports are included there. It seems they are simply not "triggered" if calling mvn verify site-deploy on the parent project.

<reporting>
  <excludeDefaults>true</excludeDefaults>
  <plugin>
    <artifactId>maven-pmd-plugin</artifactId>
    <version>3.21.0</version>
    <reportSets>
      <reportSet>
        <reports>
          <report>pmd</report>
          <report>cpd</report>
        </reports>
      </reportSet>
      <reportSet>
        <id>aggregate</id>
        <reports>
          <report>aggregate-pmd</report>
          <report>aggregate-cpd</report>
        </reports>
        <inherited>false</inherited>
      </reportSet>
    </reportSets>
  </plugin>
</reporting>

Update #2 2023-06-23: I was able to fix the PMD aggregate report creation by specifiying a unique <id> to each and every reportSet in the pom.xml files. But: This is not the complete solution, because it did not fix aggregate report creation for other plugins, e.g. Checkstyle, JXR, Surefire, Failsafe and Taglist. I took a closer look at the Effective POM and I am unable to explain this behavior. Both PMD and Checkstyle are registered for the maven-site-plugin below <build>/<plugin> in the execution with the ID default-site as a reportPlugin:

<reportPlugins>
  <reportPlugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-checkstyle-plugin</artifactId>
    <version>3.3.0</version>
    <reportSets>
      <reportSet>
        <id>checkstyle-report</id>
        <reports>
          <report>checkstyle</report>
        </reports>
      </reportSet>
      <reportSet>
        <id>aggregate-checkstyle-report</id>
        <reports>
          <report>checkstyle-aggregate</report>
        </reports>
      </reportSet>
    </reportSets>
  </reportPlugin>
  <reportPlugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-pmd-plugin</artifactId>
    <version>3.21.0</version>
    <reportSets>
      <reportSet>
        <id>pmd-report</id>
        <reports>
          <report>pmd</report>
          <report>cpd</report>
        </reports>
      </reportSet>
      <reportSet>
        <id>aggregate-pmd-report</id>
        <reports>
          <report>aggregate-pmd</report>
          <report>aggregate-cpd</report>
        </reports>
      </reportSet>
    </reportSets>
  </reportPlugin>
</reportPlugins>

Update 3 2023-06-26: I've created a Short, Self Contained, Correct Example (SSCCE) as a Git repository on GitHub.