A particularly annoying issue where an iOS device will constantly forget about a hidden wireless network. The problem seem to start after upgrading it to iOS 10. In this case it was an iPhone 5.
Resetting the network settings would seem to work for a while but it would eventually occur again. Most often it would occur after power cycling the wireless access point or if the access point lost connection to the internet. Unhiding the SSID would resolve it however that does not fix the root problem.
Here are the steps to flush out the bad configuration in the device so that it no longer forgets: Connect to the hidden wireless network as usual Go to Settings -> Wi-Fi. Click the info icon next to the name of the hidden network to which you are connected Choose Forget This Network Reboot the phone by holding the Home button and on/off button Navigate to Settings -> General -> Reset -> and choose Reset Network Settings The phone will automatically reboot Go to Settings -> Wi-Fi. Disable Ask to Join Networks Finally, reconnect to the hidden wireless network Now you should no longer have the issue of the device randomly (or not so randomly) forgetting the hidden network.
Update: Eventually the device forgot the network again. The only thing that has been a permanent solution is to broadcast the SSID, such that the network is no longer hidden.