The ever reliable R7000 just encountered a wifi problem

I have 2x r7000's running the latest firmware and I have noticed a problem that persists if the firmware is downgraded, or an open-source firmware is applied. It may have been there for some time and I didn't realise it, because these boxes are runnning as APs and are seldom used, as we generally run off a WAX218 unit.  

My wireless,  with the DHCP server is running on a firewalla gold unit (which I don't think is the cause of the problem since it provides IP addresses to clients using an NG WAX218).

With the R7000s, I can only connect one device to the wifi when it runs in AP mode, I tried putting it back into Router mode, and the same problem. I tried using a fresh tomato build and exactly the same behaviour. 

Has anyone got any ideas about what is causing this issue?