you are right - this is binding customization problem. the issue was fixed when I used customized binding file customForMaps.xml (attached) with following content:
<jxb:bindings version="2.0" xmlns:jxb="http://java.sun.com/xml/ns/jaxb"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<jxb:globalBindings generateElementProperty="false" />
</jxb:bindings>
I've added option --binding=<path/to/customForMap.xml> to the command line and finally got correctly generated xs:string properties as lava.lang.String (see attachment)