appengine-application.xml fails XML validation

Question

I have followed the instructions for configuration of multi-module appengine projects here: https://cloud.google.com/appengine/docs/standard/java/configuration-files

I copied the appengine-application.xml directly from the example provided, changing only the project name to use a Maven property. The file looks like this:

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<appengine-application xmlns="http://appengine.google.com/ns/1.0">
  <application>${appengine.app.name}</application>
</appengine-application>

My appengine-application.xml file is located in the following directory structure:

projectname-backend/
  src/...
  pom.xml

projectname-common/
  src/...
  pom.xml

projectname-ear/
  src/
    main/
      application/
        META-INF/
          appengine-application.xml
          application.xml
          MANIFEST.MF
  pom.xml

projectname-frontend/
  src/...
  pom.xml

Even after cleaning the project, Eclipse is throwing this validation error for the file:

cvc-elt.1: Cannot find the declaration of element 'appengine-application'.

Other files, particularly application.xml in the same directory, are fine. So it seems to be namespace issue but I can't find any information on what it should be - this is current example in the Google doco.

Any ideas?