I've already decided to return my EAP670 and buy another brand, but am posting this in case it helps anyone.

One of the issue I've been having is clients disconnecting "permanently".  I thought this wasn't an issue with the AP in standalone mode (not adopted to an SDN controller), but today it happened even though I am running standalone (limping along until the replacement arrives).  Prior times it has happed with IoT-type devices with no UI (smart thermostat, sprinkler controller, etc), but to day it happed with an iPad. 

It appears to happen after a client has been in "sleep" mode. When the client wakes up and tries to reconnect, the AP is rejecting the valid stored password and preventing the client from reconnecting.  On the iPad I get the "incorrect password" popup. 

iOS devices have a feature where one device can share the WiFi password over bluetooth.  Say client A is already connected, when client B attempts to connect to the network, if A is within range and already connected to the same SSID, a prompt appears on A to share the password.  Since A is connected, the password "should" be accepted when B tries to connect, but it is rejected.

To pinpoint the source of the issue I tried:

• Very carefully re-entering the known valid password (the one shown in the AP's configuration web page)
• Sharing the password from a different iOS client that is already connected to the WiFi network
• Switching off WiFi on the failing client and switching it back on
• Forgetting the WiFi network on the failing client and then attempting to connect from scratch
• Rebooting the failing client

None of these worked.  As soon as I rebooted the AP however, everything started working correctly.