Channel scanner can show odd/empty results in some cases so that's not too odd.
Only other thing I can think of is to shut down the hub and swap the antennas (they both do Zigbee and Z-Wave) and then restart. Likely won't help, since clearly your Zigbee radio is working for the devices that are still connected, functioning. But at least you can ensure both antennas are securely fastened. 
Probably should submit a Warranty case...I'd also suggest you PM your hub UID to @support_team so they can take a look at your engineering logs.
Oh - and try to reset/re-join some of your ZIgbee devices again, now that you've done the Zigbee radio rebuild/reboot steps. Does that work now?
