Electronic – Why does last data of I2C DMA transfer break

I used DMA to transfer I2C data, but I always lose the last data. hardware configuration is shown below:

  DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t)I2C1_DR_Address;
  DMA_InitStructure.DMA_MemoryBaseAddr = (uint32_t)I2C1_Buffer_Tx;
  DMA_InitStructure.DMA_DIR = DMA_DIR_PeripheralDST;
  DMA_InitStructure.DMA_BufferSize = 2;
  DMA_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
  DMA_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Enable;
  DMA_InitStructure.DMA_PeripheralDataSize = DMA_MemoryDataSize_Byte;
  DMA_InitStructure.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
  DMA_InitStructure.DMA_Mode = DMA_Mode_Normal;
  DMA_InitStructure.DMA_Priority = DMA_Priority_VeryHigh;
  DMA_InitStructure.DMA_M2M = DMA_M2M_Disable;
  DMA_Init(DMA1_Channel6, &DMA_InitStructure);
  DMA_ITConfig(DMA1_Channel6, DMA_IT_TC, ENABLE); 

The data to be transfered is:

  I2C1_Buffer_Tx[0] = 0xF0;
  I2C1_Buffer_Tx[1] = 0x55;

When a data frame has been sent, an interrupt of DMA is generated:

void DMA1_Channel6_IRQHandler(void)
{
  if (DMA_GetFlagStatus(DMA1_IT_TC6) != RESET)
  {
    DMA_Cmd(DMA1_Channel6, DISABLE);
    DMA1_Channel6->CNDTR = 2;
    DMA_ClearFlag(DMA1_IT_TC6);
    switch (mode)
    {
      case 1:
        I2C_GenerateSTART(I2C1, ENABLE);
        mode = 2;
        break;
      case 2:
        I2C_GenerateSTOP(I2C1, ENABLE);
        mode = 3;
        break;......

When I set a breakpoint at I2C_GenerateSTART(I2C1, ENABLE):

I get SDA signal from oscilloscope in this pattern:

It shows that two data have been sent through SDA:0xF0 and 0x55, however, when I set breakpoint after that code:

and I got following data:

it shows that only one data has been sent through SDA: 0xF0 and then a restart condition. so, I will lose the last data if I let the whole program run as usual.

I can only explain the phenomenon like this: DMA has already sent two data(0xF0 0x55), and then it generates an interrupt. However, those two data are still in I2C's buffer, then, in DMA's interrupt, it generates a start condition. For some reason, it breaks the final data.

Can any one give some suggestion to fix this problem or I can only set buffersize of DMA n+1 if I want to send n data.