PDDocument protect changes color palette

package protecttest;

// $ java -version
// openjdk version "1.8.0_181"
// OpenJDK Runtime Environment (build 1.8.0_181-8u181-b13-0ubuntu0.18.04.1-b13)
// OpenJDK 64-Bit Server VM (build 25.181-b13, mixed mode)

// compile with
// javac -cp pdfbox-2.0.11.jar protecttest/PdfProtector.java

// execute with
// java -cp pdfbox-2.0.11.jar:commons-logging-1.1.3.jar:. protecttest.PdfProtector


import java.io.File;
import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.pdmodel.encryption.AccessPermission;
import org.apache.pdfbox.pdmodel.encryption.StandardProtectionPolicy;

public class PdfProtector {
    public static void main(String[] args) throws Exception {
    // load and save with no protection
    PDDocument d1 = PDDocument.load(new File("Quelldatei.pdf"));
    d1.save(new File("Zieldatei-ohne-Schutz.pdf"));
    d1.close();
    // everything seems fine with the resulting PDF

    // load and save with protection
        PDDocument d2 = PDDocument.load(new File("Quelldatei.pdf"));
    AccessPermission ap = new AccessPermission();
    ap.setCanAssembleDocument(false);
    ap.setCanModify(false);
    d2.protect(new StandardProtectionPolicy("foobar","",ap));
    d2.save(new File("Zieldatei-mit-Schutz.pdf"));
    d2.close();
    // resulting file is rendered with odd colors
    }
}