After the missing bytes arrive, the ACK advances by the payload length.
Now the ACK can advance
After the retransmitted payload arrives, the ACK advances from 1001 to 1003.
1001→1003
Start plus length
The data starts at 1001 and has length 2 bytes.
1001+2=1003
Timeline shows the final ACK
The final ACK is the first server message here that asks for the byte after the two-byte payload.
final ACK=1003
Summary
Once the missing bytes arrive, the receiver can ask for the next byte after them.
next wanted=1003