Plugin classpath problem in multi module maven project

We have a pom.xml with <modules> :
<modules>
<module>./Module1</module>
<module>./Module2</module>
</modules>

In Module1 we use the define <dependencies>.
In Module2 we also define <dependencies> for maven-antrun-plugin with other <dependency>.

Problem when we display <echo message="plugin classpath: ${plugin_classpath}" />, in Module2 we have the classpath of the Module1.

The only workaround is to add specific <dependency> of Module2, in Module1 (for the maven-antrun-plugin plugin).

It looks like the plugin classpath is not updated for each Module.