This is a corner case of JBIDE-18377 which was fixed in 4.3.0.Beta1.

When you import a project with an existing jboss-deployment-structure.xml, the modules that are defined in that file are now added as jars to the build path of the project (thanks to JBIDE-18377). But, as discovered by Rob, if you only add the file after the project was created, it will not be picked up ("due to aggressive caching and a missing listener").