PCF8574A: Write, output update timing

HI, expert,

Customer have an question,

Customer want to write to data by total 20bit as below.

- Start bit (1bit)

- Slave address (0111 + A2, A1, A0 + 0 + ACK; 8bit + ACK) 

- Write Data (Data (8bit) + ACK)

- Stop bit (1bit)

However, the datasheet is described as below,

Why the output is update by second data ACK timing?

Is it meaning, customer cannot write by 20bit? If customer apply stop condition after First data byte, How will the outputs work? 

Thanks

Muk