I am not sure whether there is a small error in a Gazell packet transaction diagram, or if I have an incorrect understanding. From Gazell user guide:
If only the ACK packet sent from the Host to the Device is lost, but both the initial packet and the subsequent retransmission attempts are being successfully received by the Host, the repeated packets will be discarded by the Host, but the ACK packets will still be sent in return to the Device.
However, the accompanying diagram does not show this:
Compare this with the ESB diagram, which does show the ACK.