Rbr850 does not recover from internet drop

I'm still on fibre to the node so I use a modem in bridge mode with an Ethernet cable direct to the orbi router. Essentially the orbi authenticates the connection but the modem translates. 

Every so often the orbi drops the internet even though the modem shows a solid connection. The unit flashes white for a while then goes a solid purple requiring a reboot to restore the connection. Note that the modem does not need a reboot but I have found that rebooting both means the service stays up longer than if I just boot the orbi. 

Firmware is right up to date and it's done this since new. I've ended up building a watchdog that boots the orbi after 5 minutes of no internet but really should this be necessary?