EX8000, R9000 and the Logon problem

Let me clarify.  Here's the series of events:

1. Everything is working fine; smart connect setup on R9000, and replicated on EX8000 - coverage is great.
2. A period of time (sporadic - hours or days can pass) later, I start to notice that some mobile devices are switching to mobile data as the WiFi network (previously working fine as above) is now stating it has "no internet connection". If I force the device to continue to use the network despite the reported issue, everything continues to work (i.e. Internet connectivity is fine regardless of what the device thinks). Laptops and other devices seem unaffected at this point.
3.  Some further time passes and  I stop being able to access the EX8000's management interface by IP address or the mobile app.  Using a mobile device that is specifically connecting to my network through the EX8000 as opposed to my router, does work though.
4. Moving forward in time, all attempts to access the management interface  via the IP or the application, via wired devices on the network or wireless, fails.
5. A reboot of the R9000 seems to resolve this problem in the short term. A reboot of the R9000 and the Ex8000 seems to settle things down, and we return to (1) again.

I've noticed a few other things too; I cannot change the device name of the EX8000. I use the interface to specify a new name and it reboots as expected, but maintains the same device name.

I'm using the latest available firmware on the EX8000.

Currently, all is well, but I'm just waiting for my Wife or somebody to moan about their internet dying and me having to toggle their wireless settings, forget the network and reconnect, or even worse, bounce the router/extender. It's just not conducive to a fully functioning piece of kit.

Thoughts?