I also am having this issue on rc27.
Original Argon + 1 Xenon were paired with factory firmware. I tried to add 2 more Xenons today and they both upgraded firmware okay, The phone connects okay to the device in setup mode and I get the tick in a green circle with the device name. the app moves to the join network steps automatically where I select the existing network showing 2 devices. Then I scan the assisting device which is the Argon, connected to the internet, and I get an error saying
Error Unable to find your mesh device. Make sure the mesh device's LED is blinking blue and that it's not connected to any other devices.
The Xenon is blinking blue.
I tried closing the app on the iPhone as suggested above but that did not resolve the issue. Both Xenons behave identically.