nRF52 ESB ACK payload not sent/received with fixed payload length

Working on a project to set up a bidirectional UART to SPI link over ESB with nRF52832 (PCA10040), using SDK 13.0. Working from provided examples for PTX and PRX, I manage to send from PTX to PRX with no issues, and PTX confirms reception of ACK packet. However when using NRF_ESB_PROTOCOL_ESB as protocol setting, I am not able to get a payload with the ACK packet. Same code with NRF_ESB_PROTOCOL_ESB_DPL as protocol setting, and I am able to get the ACK packet payload. Is this intended behaviour? Are there any special settings I should be using with fixed length packet ESB to get the ACK packet payload?

I enclose both main_TX.c and main_RX.c to show which settings I am using (all default except for selective_auto_ack = true).RX_TX.zip

I can provide the full standalone project folder with all components if needed (all SDK 13.0 components)