First: this is not a hardware problem, the same thing happens to me on two different phones. Factory reset, obviously, does not help (tried that, of course). Not a Android 8.1 problem either, since I had that on my P6 for a long time, with no random reboots.
Event logger app gives no indication of what the problem is. Phone is not in direct use when it happens. It's not the Fitbit app, which I know can be a problem. Not France24 news app either, even though that doesn't work as it should (1 in 40 notifications come trough, roughly, just like with Skype, but it's more normal with Skype to act like that).
Any other apps that might be the culprit? The Messages (SMS/MMS) app is taking more out of the battery than it should for the lack of usage, but I can't remove it. Same with Phone app.