PNG specification mentions limit of keyword length for different chunks.
But in PNGImageWriter we dont check for keyword length and we just populate the data.

Using below test code i am able to create tEXt chunk with keyword greater than 80.

Test code :
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.awt.Color;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.Iterator;
import javax.imageio.ImageTypeSpecifier;
import javax.imageio.ImageWriter;
import javax.imageio.ImageIO;
import javax.imageio.ImageWriteParam;
import javax.imageio.metadata.IIOInvalidTreeException;
import javax.imageio.metadata.IIOMetadata;
import javax.imageio.metadata.IIOMetadataNode;
import javax.imageio.stream.ImageOutputStream;
import javax.imageio.IIOImage;

public class MaxLengthKeywordWriteTest {

    private static BufferedImage img;
    private static ImageWriter writer;
    private static ImageWriteParam param;
    private static IIOMetadata metadata;

    private static void initialize(int type) {
        int width = 1;
        int height = 1;
        img = new BufferedImage(width, height, type);
        Graphics2D g2D = img.createGraphics();
        g2D.setColor(new Color(255, 255, 255));
        g2D.fillRect(0, 0, width, width);
        g2D.dispose();

        Iterator<ImageWriter> iterWriter =
                ImageIO.getImageWritersBySuffix("png");
        writer = iterWriter.next();

        param = writer.getDefaultWriteParam();
        ImageTypeSpecifier specifier =
                ImageTypeSpecifier.
                        createFromBufferedImageType(type);
        metadata = writer.getDefaultImageMetadata(specifier, param);
    }

    private static void createTEXTNode()
            throws IIOInvalidTreeException {
        IIOMetadataNode tEXt_Entry = new IIOMetadataNode("tEXtEntry");
        // Use keyword of length 80
        tEXt_Entry.setAttribute("keyword", "Authored" +
                "AuthoredAuthoredAuthoredAuthoredAuthoredAuthored" +
                "AuthoredAuthoredAuthoredAuthored");
        tEXt_Entry.setAttribute("value", "");

        IIOMetadataNode tEXt = new IIOMetadataNode("tEXt");
        tEXt.appendChild(tEXt_Entry);
        IIOMetadataNode root = new IIOMetadataNode("javax_imageio_png_1.0");
        root.appendChild(tEXt);
        metadata.mergeTree("javax_imageio_png_1.0", root);
    }

    private static void writeImage() throws IOException {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        ImageOutputStream ios = ImageIO.createImageOutputStream(baos);
        writer.setOutput(ios);
        writer.write(metadata, new IIOImage(img, null, metadata), param);
        writer.dispose();
        baos.close();
    }

    private static void writePNGTEXTChunk() throws IOException {
        initialize(BufferedImage.TYPE_BYTE_GRAY);
        // Create tEXt node with text length 0
        createTEXTNode();

        writeImage();
    }

    public static void main(String[] args) throws IOException {

        // read PNG image where tEXt chunk's text length is 0
        writePNGTEXTChunk();
    }
}