Odd for sure. I don‘t see that on my 6s Plus. I don't think it's wide spread. Obviously some do have the issue though. I would first try Reset All Settings.
Reset All Settings
Go to Settings >General >Reset >Reset All Settings. This resets to default everything in Settings, including notifications, alerts, brightness, and clock settings like wake up alarms.
If that doesn't work, you may need to restore your phone as new. Not from a back up. It's a PITA, but it fixes many issues. I did it 3 days after I got my 6s Plus in September.
This too:
https://discussions.apple.com/thread/5431334?tstart=0