iOS 11: iPhone freezes/shuts down since update Ever since iOS 11 came out, my iPhone 6s intermittently "freezes" or "shuts down". It's difficult to phrase it exactly, because I typically see some different behaviours:


The phone freezes and is unresponsive, but it's still active (e.g. playing music). The screen is black and lock/unlock button is unresponsive. Frequently happens after pressing the lock button once, after which it will not respond to an unlock request.
The phone suddenly "semi-shuts down". Music playing will stop suddenly and Bluetooth connections such as AirPods will be lost. The phone is either entirely black or "graphically black" with a spinning wheel for about 20–30 seconds. It does not appear to be a full shutdown, because no reboot is required (i.e. the Apple logo isn't displayed). The phone returns to the Lock Screen after the black screen or spinning wheel.


It should be noted that my phone has battery problems described here and here, which is to say that in iOS 10 the Phone would suddenly completely die at 20%. While the issue could definitely be related to the faulty battery, it does not appear to be similar, and these new shutdowns occur also at near-fully loaded battery levels.

Anecdotally, I don't have much to report other than that the Lock Button seems to trigger a freeze sometimes, but as stated above shutdowns can happen by themselves without any particular action by me (except streaming music).

Note: This problem did not occur in iOS 10 or similar versions with the very same phone.



Edit: I have not replaced any part of the iPhone, e.g. I don't use a third-party replacement screen. The iOS 11.0.3 update that was intended to address issues relating to replacement screens, did not help.
There are lots of issues right now with iOS 11, especially with the iPhone 6S. A quick look at the iFixit Answers Forum will convince you of that. Of course, these are just a tiny percentage of all iPhone owners so it's not like there is a pandemic going on. If your screen has been replaced with an aftermarket screen in the past, this may be the source of the problem. 

This blog post explains:

The problem appears to be caused by the software driver of the Touch Controller IC that is embedded into the display assembly. Aftermarket screens produced with Hancai or Tianma LCD's using the Aichungjia (a Flex/IC manufacturer) "copy" driver appear to be incompatible with iOS 11. This driver is said to be unable to handle the iOS configuration and therefore shutting down and causing the lack of Touch functionality. OEM or Original grade iPhone 6S and iPhone 7 series replacement screens are not affected by this problem.

I don't believe it is battery related although you should check this Apple official website to see if you are elegible for a free battery replacement. Unfortunately, it is now too late to downgrade to iOS 10 as Apple have stopped signing those versions.