parent with multi-type mojo children does not build ant mojo

I've a parent with two childeren. Both are mojos. One is a java mojo, the other is an ant mojo.
When I run mvn clean install in the parent, the ant mojo is not build correctly.
To reproduce use the project in the attached tgz file:
tar -zxvf antmojobug.tgz
cd antmojobug
mvn clean install.

Observe that ant-child/target/classes only contains a META-INF directory. The file test.build.xml is missing.
I notice that the mvn output gives:
...
[INFO] [plugin:descriptor]
[INFO] Using 2 extractors.
[INFO] Applying extractor for language: java
[INFO] Extractor for language: java found 0 mojo descriptors.
[INFO] Applying extractor for language: bsh
[INFO] Extractor for language: bsh found 0 mojo descriptors.
...

When I run mvn clean install in the ant-mojo directory I get a proper mojo and the output is :
...
[INFO] [plugin:descriptor]
[INFO] Using 3 extractors.
[INFO] Applying extractor for language: java
[INFO] Extractor for language: java found 0 mojo descriptors.
[INFO] Applying extractor for language: bsh
[INFO] Extractor for language: bsh found 0 mojo descriptors.
[INFO] Applying extractor for language: ant
[INFO] Extractor for language: ant found 1 mojo descriptors.
...

When the order of modules in the parent pom is reversed, the ant mojo is correctly build.