iPhone 6s Plus Delayed Missed Call Notification On Lock Screen

I'm facing this strange problem on my iPhone 6S Plus. Missed call notification appear on lock screen only after 8-10 seconds of getting the missed call. It's been 2 months and even the Apple Care has not been able to solve the problem. All they say is that there's some problem with my data and that my phone works fine without my backup. Also I've tried restoring the same backup on my old iPhone 6 and it works perfectly fine. How is it that it works fine on my iPhone 6 but not on the iPhone 6S Plus if the problem is with the data? I told Apple Care in India to swap my device since it's under warranty but they are not ready to swap. I'm really frustrated with it. They don't give me any solution neither do they swap the device. Can anyone help me with the solution?

If I restore the phone & restore the backup it works for a day , then again back to no delayed missed call notification. Once I even deleted "whatsapp" and immediately the missed calls started to appear. But then again the next day it was back to normal (delayed missed call notification)