Static variable resetting in C after value 0x0c

Question

I am trying to read continuous data from remote device and I have static variable declared to receive and send ACK . Payload of 0 and 1 holds the sequence number of the data I am getting from remote device .

The problem I have is with variable fragment_num. After it reaches 0x0c it is resetting back to 0.

Its Free RTOS application . Are there any obvious reasons for a static variable to reset to 0 or is there any problem with my code ? Thanks

#define INTIAL_FRAGMENT 0x00
static uint8_t length;
static uint8_t fragment_num ;
uint8_t image[128];
download ()
{
  if(((payload[1] << 8) | (payload[0])) == INTIAL_FRAGMENT)
{
  memset(image , 0,128);
  memcpy(image , payload,(len));
  info_download();
  length = len;
  fragment_num +=1 ;
 
}

   if(((payload[1] << 8) | (payload[0])) == fragment_num)
{
  memcpy((image+length+1) , payload,(len));
  length += len;
  fragment_num ++;
  info_download();
}