The problem, as far as I can see, is in{{ ThrowableProxyConverter.subjoinExceptionMessage}} which does

buf.append(tp.getClassName()).append(": ").append(tp.getMessage())

while Throwable.toString() does:

String s = getClass().getName();
String message = getLocalizedMessage();
return (message != null) ? (s + ": " + message) : s;

The difference is that if message is null, it is not printed. However, Logback prints it.