So what I've determined is we've all had exactly the same issue, no one knows what it is, and Apple hasn't replied here but I went ahead and did a support chat and here's what I got:
- Okay, please be assured that your iPhone is secured and the iOS that you have on your iPhone has the best security in the market.
- Let me explain a device restart can be caused by the App that you are using at that time or any system issue.
- All the discussion and the feedback are checked by the Apple engineers on a regular basis. Also any issue that is occurring or needs to be checked is also escalated to the team so they are aware of any system issue and work on it to resolve it.
That's as far as I got.