Gradle :idea report problem

What's wrong

In build.gradle user adds these lines:

sourceSets {
  custom {
    java {
      srcDir 'src/custom/java'
    }
  }
}

Idea recognise srs/custom/java as source root. But :ideaModule task does not add src/custom/java as sourceFolder.

Result file from :ideaModule task gradle-idea-report-problem.iml:

<?xml version="1.0" encoding="UTF-8"?>
<module relativePaths="true" type="JAVA_MODULE" version="4">
  <component name="NewModuleRootManager" inherit-compiler-output="true">
    <exclude-output/>
    <orderEntry type="inheritedJdk"/>
    <content url="file://$MODULE_DIR$/">
      <sourceFolder url="file://$MODULE_DIR$/src/main/java" isTestSource="false"/>
      <excludeFolder url="file://$MODULE_DIR$/.gradle"/>
      <excludeFolder url="file://$MODULE_DIR$/build"/>
    </content>
    <orderEntry type="sourceFolder" forTests="false"/>
  </component>
  <component name="ModuleRootManager"/>
</module>

How to reproduce

1. Clone this project
2. Open in IDEA
3. Run rm gradle-idea-report-problem.iml && ./gradlew ideaModule
4. Compare gradle-idea-report-problem.iml report with File > Project Structure

Questions

1. That difference between report and idea logic is expected behaviour?
2. If yes, How can I check in unit test what the IDEA knows about the custom sourceSet?