Some of the devices, affected by the problem with the battery, where the phone turns off by its own, tend to do this behavior on iOS versions after 10.2. This was actually the "fix" from Apple's side...
Did you have some spontaneous shut downs while on 10.2?
Try to update to 10.3.3, it should resolve your problem
Yes, I have 6S but I am still on 10.2. I didn't want to take the risk to update to 10.3, because I wasn't sure if this isn't going to slow my phone down
Yes, I got my previous device replaced, because of battery - it was shutting down unexpectedly at 20-30% power left. My current device is ok
I had the same problem. Its pretty common (check official apple support). The only way to fix this is to change your battery to new. It will help you for sure.