Increasing the size of a RandomAccessFile via a MappedByteBuffer with the "map" function changes file pointer in JDK 8

ADDITIONAL SYSTEM INFORMATION :
Windows 10 Professional
JDK 1.8.0_172
JDK 10.0.1

A DESCRIPTION OF THE PROBLEM :
Increasing the size of a RandomAccessFile via a MappedByteBuffer with the "map" function results in overwriting the already existing part.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Install JDK 1.8.0_172 (short: JDK8)
2. Install JDK 10.0.1 (short: JDK10)
3. Modify the "path" and "fileName" constants in the class MMFFail
4. Compile the given code with JDK8 (<path to JDK8>\\javac.exe -cp ... MMFFail.java)
5. Run the .class file with JDK8 (<path to JDK8>\\java.exe -cp ... MMFFail) [result: File with 1,048,576 Byte]
6. Run the .class file with JDK10 (<path to JDK10>\\java.exe -cp ... MMFFail) [result: File with 524,288 Byte]
7. Compile the given code with JDK10 (<path to JDK10>\\javac.exe -cp ... MMFFail.java)
8. Run the .class file with JDK10 (<path to JDK10>\\java.exe -cp ... MMFFail) [result: File with 524,288 Byte]

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Running MMFFail should produce two similar files (with a size of 1,048,576 Byte) independent of running it with either JDK8 or JDK10.
ACTUAL -
Running MMFFail with JDK8 produces a file with a size of 1,048,576 Byte
Running MMFFail with JDK10 produces a file with a size of 524,288 Byte.

---------- BEGIN SOURCE ----------
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;

public final class MMFFail {

    //Path to the generated file
    private static final String path = "C:\\tmp\\";
    private static final String fileName = "myMem";

    public static void main(String[] args){

        File myFile = new File(path+fileName+"_"+System.currentTimeMillis()+".txt");

        if(myFile.exists()){
            myFile.delete();
            System.out.println("Delete old file.");
        }

        try {
            // Create a RAF
            RandomAccessFile raf = new RandomAccessFile(myFile, "rw");

            // Allocate 524288 byte on the disc
            MappedByteBuffer buf = raf.getChannel().map(FileChannel.MapMode.READ_WRITE, 0, 524288);

            //Fill with 'a'
            while (buf.remaining()>=1){
                buf.put((byte) 'a');
            }
            buf.force();

            // Allocate additional 524288 byte on the disc, at the pointer position
            // expect: 1048576 byte allocated on the disc
            // result for 1.8.0_172: 1048576 byte allocated
            // result for 10.0.1: 524288 byte allocated
            buf = raf.getChannel().map(FileChannel.MapMode.READ_WRITE, raf.getFilePointer(), 524288);

            //Fill the 524288 byte with 'b'
            while (buf.remaining()>=1){
                buf.put((byte) 'b');
            }
            buf.force();

            //Close the RAF
            raf.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

---------- END SOURCE ----------

FREQUENCY : always