I haven't had issues like the firmware or anything with nougat. Upgraded to beta thru labs and the stable oreo.
This problem started only after installing some app, not sure which one. In any case I took the advice above and it seems to reduce the frequency of the nuisance. I had to endup doing a hard reset and not restore everything from backup.
Rob if you have recently installed the jan security patch then maybe it could be to do with that. It might need a hard reset. No point in taking the beta unless you want to risk loosing the lot in case you have to revert back. I would advice a sound backup using sms back up and restore which does contacts and call logs and if you want the lot use Gcloud which is pretty good. I am not sure about the app data as I had downloaded each app individually for the all the apps that I know I had and then I am excising caution,
It should pretty much fix whatever was wrong with it. Just be mindful not to install from google backup when you login using your google account.(Just in case)
As for your query yes wifi, mobile data and bluetooth always on although this issue occurs regardless on which data is getting used.
AFAIK if the wifi is connected, even without internet access the mobile data shouldn't be used and while I am using chrome or some apps like ebay, play store, or even the google app on the left of the home screen I get the" network error please try again" balloon on the bottom of the screen. ----> internet is being used on wifi and still network error, Idle phone on home screen and network error!!! So I ended up with a hard reset on oreo 8.1 beta setup as new device.
I just hope there are not any major issues with the beta if not another hard reset is on my cards *dreading it*