testOutputDirectory is ignored during Maven build

I am working on a Xtext/Xcore/Xtend/Maven/Tycho project. My problem is that the Xtend test classes are generated into the wrong directory during the Maven build. When the Xtend compiler is triggered by the workspace builder, the test sources are generated into the correct directory. This leads to duplicate test sources.

My Maven setup:

<plugin>
	<groupId>org.eclipse.xtend</groupId>
	<artifactId>xtend-maven-plugin</artifactId>
	<executions>
		<execution>
			<goals>
				<goal>compile</goal>
				<goal>testCompile</goal>
			</goals>
                     <configuration>
                         <writeTraceFiles>false</writeTraceFiles>
                         <outputDirectory>${project.basedir}/src/main/xtend-gen</outputDirectory>
                         <testOutputDirectory>${project.basedir}/src/test/xtend-gen</testOutputDirectory>
                     </configuration>
		</execution>
	</executions>
</plugin>

When run in Maven, the test sources are wrongly generated into src/main/xtend-gen; when run by the Eclipse builder, they are correctly generated into src/test/xtend-gen.

Start at the xtend-maven-plugin compile and testCompile goals and trace how outputDirectory and testOutputDirectory are applied during a Maven build. Reproduce the configuration from the issue, then verify that test sources are generated under src/test/xtend-gen rather than src/main/xtend-gen, without duplicate sources.