SPI Slave SPIS rx via ENDRX event (no CS toggle) and re-config buffers for tx ACK/NAK without interim CS toggle

Question

Hi 
 My application requires that SPI CS is NOT TOGGLED within a transaction, only at either side 
 My Transaction consists of: 
 1 - Slave sets up buffer ready for Rx and waits: nrf_drv_spis_buffers_set(&spis, NULL, 0, (uint8_t*)&sProteusMsg, 8); while(!spis_xfer_done) {} 
 2 - Master Sets CS Low 
 3 - Master clocks in 8 bytes to Slave, slave knows RX complete via ENDRX, NOT via CS toggle 
 4 - Slave re-configures buffers for a single byte Tx/Rx: nrf_drv_spis_buffers_set(&spis, &status, 1,&dummy,1); 
 5 - Master reads the single byte from the slave 
 6 - Master Sets CS High 
 The above FAILS - it looks like it fails on the SPI slave at step 4 - the re-configure of the buffer does not 'take' 
 If I flip CS High/Low on the master between steps 4 and 5, everything works. I am guessing this is down to the acquisition of the SPIS semaphore, I need to 'mimic' whatever it is that is occurring on the slave when CS is toggled - I need to do this when I receive the ENDRX event for the initial 8 bytes transferred master-->slave. 
 I have tried many many things, all to no avail, and am hoping for some assistance. I need to know what happens on the slave when CS is flipped high/low between the Tx and the Rx so that I can mimic this behaviour somehow? 
 NB: To make the ENDRX event work for SPIS, I had to modify the code. My mods are in this thread: https://devzone.nordicsemi.com/f/nordic-q-a/41249/spi-slave-comms-using-byte-count-eg-rx-buffer-full-rather-csn-chip-select-to-indicate-end-of-spi-transaction 
 Kind Regards 
 
 Nigel 
 My SPIS callback function below: 
 void spis_read_event_handler(nrf_drv_spis_event_t event)
{
 if (event.evt_type == NRF_DRV_SPIS_BUFFERS_SET_DONE)
 {
 NRF_LOG_INFO("NRF_DRV_SPIS_BUFFERS_SET_DONE");
 }
 
	
 if (event.evt_type == NRF_DRV_SPIS_ENDRX_DONE)
 {
 if (event.rx_amount > 0 || event.tx_amount > 0)
 {
 NRF_LOG_INFO("ENDRX --> Rx Byte Count: %d",event.rx_amount);
 NRF_LOG_INFO("ENDRX --> Tx Byte Count: %d",event.tx_amount);
 
 spis_xfer_done = true;
 }
 }
	
 if (event.evt_type == NRF_DRV_SPIS_XFER_DONE)
 {
 if (event.rx_amount > 0 || event.tx_amount > 0)
 {
 NRF_LOG_INFO("Via CS LO-->HI Rx Byte Count: %d",event.rx_amount);
 NRF_LOG_INFO("Via CS LO-->HI Tx Byte Count: %d",event.tx_amount);
 spis_xfer_done = true;
 }
 }
}