Over the past couple of iPhone versions users have complained of
“unexpected” shutdowns of their devices. Some iPhone 6, 6S, 6 Plus
and 6S Plus devices could basically go dark unexpectedly, forcing
a user to have to plug them into an outlet to get them to power
Apple has been working on this very annoying bug and it says it
has come up with a fix of sorts that should mitigate the problem
on a majority of iPhone 6 and iPhone 6s devices. The fix is
actually already on your iPhone if you have installed iOS 10.2.1
— something that around 50 percent of iOS users have already
done. After letting the fix simmer on customer devices, Apple now
has statistics to share on how it has improved the issue, citing
80 percent reduction on iPhone 6s and 70 percent reduction on
iPhone 6 devices.
I used to see this occasionally on my 6S, but as Panzarino notes, it was never a problem with the 7.