build-helper-maven-plugin on profile doesn't work

[provide a description of the issue]

Dear support,
I am trying to use the profiles in a POM to add extra source directory.
I am using a plugin which generate java code in a custom directory target/generated-sources/plc4x and for telling VSC that this directory has sono code I would like to use build-helper-maven-plugin.

If I use the plugin inside the POM it works:

  <build>
    <plugins>
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>build-helper-maven-plugin</artifactId>
        <executions>
          <execution>
            <id>add-source</id>
            <phase>generate-sources</phase>
            <goals>
              <goal>add-source</goal>
            </goals>
            <configuration>
              <sources>
                <source>target/generated-sources/plc4x</source>
              </sources>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>

In this way VSC add correctly the new directory in the .classpath file:

        <classpathentry kind="src" output="target/classes" path="target/generated-sources/plc4x">
                <attributes>
                        <attribute name="optional" value="true"/>
                        <attribute name="maven.pomderived" value="true"/>
                </attributes>
        </classpathentry>

but if I try to use profiles to handle the plugin just in some circumstances, it doesn't work:

<profiles>
<profile>
      <id>vsc</id>
      <activation>
        <os>
          <family>mac</family>
        </os>
      </activation>
            <build>
        <pluginManagement>
          <plugins>
            <plugin>
              <groupId>org.codehaus.mojo</groupId>
              <artifactId>build-helper-maven-plugin</artifactId>
              <executions>
                <execution>
                  <id>add-source</id>
                  <phase>generate-sources</phase>
                  <goals>
                    <goal>add-source</goal>
                  </goals>
                  <configuration>
                    <sources>
                      <source>target/generated-sources/plc4x</source>
                    </sources>
                  </configuration>
                </execution>
              </executions>
            </plugin>
          </plugins>
        </pluginManagement>
      </build>
</profile>
</profiles>

In this way the entry in the .classpath file is not present.

Environment

• Operating System: Mac Os 11.2
• JDK version: openjdk version "13.0.2" 2020-01-14
• Visual Studio Code version: 1.53.0

Current Result

I can't use profiles to handle the build-helper-maven-plugin

Expected Result

To have the the path entry in the classpath in both the situation

Regards,
Stefano Bossi

Reproduce the two POM configurations on macOS with the listed JDK and VS Code versions, comparing the generated .classpath entry when build-helper-maven-plugin is under build versus an OS-activated profile. Start with Maven profile handling and VS Code Java project import; done means the generated-source path appears in .classpath in both reported situations.