I found something. If you choose a stock ringtone as default ringtone and reboot your phone the selection is saved.
But, if you choose a custom ringtone that you put on your phone (internal or sd card) and reboot. Your selection is not saved and the default ringtone "Nokia Tune" is selected as default.
Otherwise, no problem detected if you choose a custom ringtone for a specific contact.
I have the exact same problem on 8.0. Hoping 8.1 will fix it (if it ever arrives).
I tried wav, aac, mp3, oog. Same result.
If you reboot and go immediately in ringtones settings you can see your custom ringtone is still selected. If you stay on that setting screen and make a call with another phone, the ringtone will be your custom ringtone.
Then, exit the ringtones settings then go back and you can see the ringtone selection reset itself. "Nokia Tune" is back.
It's obviously a bug.
The only solution for now is to not turn off your phone. But if your battery is dead, or just want to turn off your phone when you sleep, this bug is kinda annoying.
I have no idea if the 8.1 update correct this. I'm stuck to the 8.0.
Okay... apparently my post was censored for no reason. So I said I tried wav, mp3, ogg and aac. Same result.
If you go quickly on the ringtone setting after a reboot you can see your custom ringtone is still selected. And if you make a call on a another phone, the Nokia 7 Plus will use your ringtones. But, after exit the settings and go back, it reset itself the selection and put back "Nokia Tunes".
The only solution is to not turn off the phone. Yeah I know...
So it is obviously a bug to not saving your ringtone setting after a reboot.
I have the same issue as well on my Nokia 7 Plus. Another interesting thing is, that a family member of mine who has very recently bought the new Nokia 6 (2018) has the exact same issue as well. So it appears to affect that model as well.
I'm trying a few things to see if I can make the ringtone stick after a restart. No luck so far, but if I do find a way round this bug, I'll report back :)
The only way (apparently) to report this bug is contacting Nokia via the chat support (because we're only user here). So everyone need to contact Nokia to talk about it. If I trust the last Nokia staff I chat with, they keep track of our bug report.