So after PMing with @armor and Particle enterprise support, we went back to the customer with Photon Cloud Debug firmware modified with SerialLogHandler (LOG_LEVEL_ALL).
We tried multiple things but kept getting wiced_join_ap_specific(), result: 1006 on the required WiFi. The Photon softAP stopped responding afterwards without a complete clearing of WiFi.
With a network analyser, we realised the problem was embarrassingly simple…
The required WiFi had both 2.4GHz and 5GHz bands on the same SSID.
As we all know 5GHz does not play nice.
This poses a bit of a problem, because even though it is rare for an AP to have both bands with the same SSID, the perception for this customer is that the device does not work. I will request them to change the SSID of one of the bands but I know what the answer will be (as it’s a large company with a few hundred phones/laptops connected at any one time).
Best solution: Is there a way to isolate and connect to the specific AP MAC address?
P.S. Here is a post posing a similar question but without a solution.