What is happening here? Is it hardware or software issue? It is really annoying. I’ve been using my iPhone 6s running iOS 11.3.1 for about 2 years, and this problem came with iOS 11 update. So I think it’s firmware problem. Any help would be appreciated? Force restart isn’t an option.
I'm not sure why a forced restart isn't an option unless you mean you have already tried it. If so, the next steps are usually a restore from backup followed by a reset:
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
If you don't like either of those options (and still suspect it is a hardware/firmware problem), then I suggest making an appointment at a Genius Bar in an Apple Store near you or contacting Apple support online. Either way:
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My iPhone 6s won't wake up right away after locking the screen
