Trying to pass an extra attribute to MapItem leads to a null value in an unrelated class when that value is an attribute with a location specified

We have existing XML files, e.g. for configuration, that use a Map for holding parameters. Besides name and value there is also a "mandatory" field which is not mapped or used.
The existence of this field mysteriously leads to a null value elsewhere. This only happens if that value comes from an attribute with the "location" attribute in it's mapping.
Version 1.2 did not have this error. I have attached a small test project to demonstrate this strange behavior.