Multiple `maven-jar-plugin` executions causes compilation issue

I have a multi-module Maven project:

• Project root
  • Module: main-application
  • Module: additional-module - depends on main-application

I'm using maven-jar-plugin to compile additional JAR files during when main-application is built, based on the contents of some directory:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-jar-plugin</artifactId>
    <executions>
        <execution>
            <id>additional-jar</id>
            <phase>test-compile</phase>
            <goals>
                <goal>test-jar</goal>
            </goals>
            <configuration>
                <testClassesDirectory>${project.build.directory}/extra-classes-input</testClassesDirectory>
                <outputDirectory>${project.build.directory}/extra-classes-output</outputDirectory>
            </configuration>
        </execution>
    </executions>
</plugin>

When I clean test-compile the project, I get a compilation error in additional-module, where it can't find any of the test-scoped classes in main-application:

• package com.main.application does not exist
• cannot find symbol in additional-module's test classes referencing anything from main-application's tests

More specifically, the testClassesDirectory property of maven-jar-plugin is the inflection point between compilation failing and succeeding:

• When absent, compilation is fine
• When set, compilation fails

I'm trying to compile an additional JAR containing code from extra-classes-input, but it looks instead I'm amending the test classes that additional-module is trying to access.

I don't want any of the contents of the extra-JAR on the classpath so I can't add additional modules for the JAR I want to build.

What could I be doing wrong?

Because I was not specifying different classifiers, the outputs were overwriting each other - there was a warning I missed to that effect.