I suspect, from examination of the code, that the XMLSignatureSigningStage's includeX509SubjectName property doesn't do anything, and may cause a run-time error.
This is because it adds a String (the the subject name in a particular format) to the array of KeyInfo items to be added to the signature:
keyInfoItems is a List<Object>. which works with the Java 8 API (expects a raw List) but the contract actually corresponds to a List<XMLStructure> and is so represented in Java 9.
Either the string needs to be turned into an appropriate XMLStructure subtype, or it should actually be added to the x509Data as the other elements in this method are.
There is no unit test for this case, which is probably how we should start out.