https://docs.oracle.com/javase/specs/jls/se8/html/jls-3.html#jls-3.3 specifies processing of Unicode escapes which happens before the source is being parsed. Most notably it contains the following:

If an eligible \ is followed by u, or more than one u, and the last u is not followed by four hexadecimal digits, then a compile-time error occurs.
            

This is very easily triggered by a model like:

module def {
  namespace def;
  prefix def;

  leaf def {
    type string;
    default "\\u@\\h\\M% ";
  }
}

where the snippet emitted for javadoc purposes will end up containing what looks like an eligible escape:

/**
 *
 * <p>
 * This class represents the following YANG schema fragment defined in module <b>def</b>
 * <pre>
 * module def {
 *   namespace def;
 *   prefix def;
 *   leaf def {
 *     type string;
 *     default "\u@\h\M% ";
 *   }
 * }
 * </pre>
 *
 */