File storing issue (LittleFS flash file system)

Edge Systems
1

Hello everyone, I guess I missing something, but I have a problem with file storage. After the reboot, I can not able to read the file which I created before the reboot. My code:

// ===============
// Step 1 - loading saved data if it exists
// ===============
  struct stat st;
  stat("/plate.dat", &st);
  long size = st.st_size;
  if (size!=310200) // Check that file has a correct size
  {
    Serial.printlnf("%s: Error: can not load save data, file size = %d", __func__, size);
    return;
  }

// .....

  int rFd = open("/plate.dat", O_RDONLY);
  if (rFd==-1)
  {
      Serial.printlnf("%s: Error Open file! ", __func__);
      return;
  }

  for (int i=0; i<MAX_X*MAX_Y/8/100; i++ )
  {
    read(rFd, buff, SRAM_BUFF_LEN);
    // ...
  }
  close(rFd);

// ===============
// Step 2 - save data to file
// ===============

  int rFd = open("/plate.dat", O_RDWR | O_CREAT | O_TRUNC);
  if (rFd == -1)
  {
    Serial.printlnf("%s: Error Can not open file!", __func__);
  }

  for (int i=0; i<MAX_X*MAX_Y/8/100; i++ )
  {
      // ...
        write(rFd, buff, SRAM_BUFF_LEN);
  }
  fsync(rFd);
  close(rFd);

// And theck the file size right after it close:
  struct stat st;
  stat("/plate.dat", &st);
  long size = st.st_size;
  Serial.printlnf("%s: Save to file %d bytes", __func__, size);

The first run:
Step 1 - file not exist,
Step 2 - file wrote without any errors. The file size is 310200 bytes.
After reboot:
Step 1 - file exists, but the file size is 19 bytes (instead of 310200 bytes).
Step 2 - file wrote without any errors. The file size is 310200 bytes.

So I can not realize what could happen with the file after reboot, that its size is changed. Just in case, am I right that the file system storing in nonvolatile memory?

My device:
Device OS: 2.0.0
Type: Asset Tracker