Why does Jenkins only push the parent Maven project to Sonar?

Why does Jenkins only push the parent Maven project to Sonar?

I have a multi-module Maven project looking like this
/parent
/module-a
/module-b
parent acts as aggregator and actual parent project for the modules. I
have a Jenkins job that builds parent and a Sonar post-build step without
changes to the settings. During the build, which runs from /parent,
everything is fine, all the modules are being built. In Sonar however, I
only see parent afterwards, without any submodules. Why is that so? Is it
because of the flat folder structure or is there another reason?