To simplify the problem, we have an encrypted channel set up with two ANT devices, which works.
On the Master side we use a nrf52 loaded with Network Processor firmware.
On the Slave side we use a nrf52, calling the softdevice api's directly.
We modified the Master channel to set:
Assign Channel - Extended Assign
High Priority Search Timeout - 12
Frequency Agility Config - for frequencies 2403, 2439, 2475
We modified the Slave channel to set:
Assign Channel - Extended Assign
High Priority Search Timeout - 10
Low Priority Search Timeout - 2
Frequency Agility Config - for frequencies 2403, 2439, 2475
On the Master side we receive TX failed events on that channel until the Slave comes online, however an encryption negotiation success event is never fired.