getContentDispositionFilename uses encodedFilename in both parameters where decoded one should be used

/**
 * Returns the filename formatted for inclusion in a Content-Disposition
 * header. Includes both a plain version of the name and a UTF-8 version
 *
 * @since 7.4.8
 * @param filename
 *            The filename to include
 * @return A value for inclusion in a Content-Disposition header
 */
public static String getContentDispositionFilename(String filename) {
    String encodedFilename = EncodeUtil.rfc5987Encode(filename);

    return String.format("filename=\"%s\"; filename*=utf-8''%s",
            encodedFilename, encodedFilename);

Observe that you use encodedFilename in both cases instead of using filename in one case and encodedFilename in the other.

Start at getContentDispositionFilename(String filename) and inspect how EncodeUtil.rfc5987Encode(filename) is used to build the Content-Disposition value. Verify that the plain filename parameter uses the original filename while the UTF-8 parameter uses the encoded value, then run the relevant project tests or add coverage for both parameters.