Namespace prefix is not included after first sibling

Two schemas car.xsd and engine.xsd are defined with two different targetNamespaces.
car.xsd imports engine.xsd and defines some elements. The elements are defined either in car.xsd or in engine.xsd.

Car.xsd is compiled using the buildCarAnt.xml file.
The generated classes are used in the CarTester.java program.

The result of CarTester.java is as follows

<car:car xmlns:car="http://car.aflac.com">
<car:engine>
<engine:cylinderCount xmlns:engine="http://engine.aflac.com">6</engine:cylinderCount>
<engine:cylinder>
<engine:diameter>12.44</engine:diameter>
<engine:volume>750.45</engine:volume>
<engine:valveCount>32</engine:valveCount>
</engine:cylinder>
<engine:material>
<engine:name>Al</engine:name>
<engine:percentage>90.14</engine:percentage>
</engine:material>
</car:engine>
<car:body>
<car:style>Coupe</car:style>
<car:weight>3454.54</car:weight>
<car:doorCount>2</car:doorCount>
</car:body>
</car:car>
Time to unmarshall it
[Fatal Error] :4:22: The prefix "engine" for element "engine:cylinder" is not bound.
javax.xml.bind.UnmarshalException: The prefix "engine" for element "engine:cylinder" is not bound.
at org.apache.ws.jaxme.impl.JMUnmarshallerImpl.unmarshal(JMUnmarshallerImpl.java:103)
at org.apache.ws.jaxme.impl.JMUnmarshallerImpl.unmarshal(JMUnmarshallerImpl.java:85)
at com.aflac.quote.client.simple.CarTester.main(CarTester.java:73)
Caused by:
org.xml.sax.SAXParseException: The prefix "engine" for element "engine:cylinder" is not bound.
at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
at org.apache.ws.jaxme.impl.JMUnmarshallerImpl.unmarshal(JMUnmarshallerImpl.java:98)
at org.apache.ws.jaxme.impl.JMUnmarshallerImpl.unmarshal(JMUnmarshallerImpl.java:85)
at com.aflac.quote.client.simple.CarTester.main(CarTester.java:73)
--------------------------------------------------------------------------

<engine:cylinder> and <engine:material> tags are missing the attribute

xmlns:engine="http://engine.aflac.com"

I mark this as Blocker because my client cannot unmarshall the XML and I cannot find a workaround.