Strange inconsistent framerate drop on iOS portrait orientation

I am using Unity 2017.3.1p1 and there is a very strange framerate drop on iOS, sometimes the game runs smoothly, but if I change the volume of the device using the left side buttons and after the volume icon that shows up on the center of screen on iOS fades out, framerate suddenly drops very low. Then framerate comes back to normal randomly. And again framerate drops if the go to the control center and come back to the game. There is no consistency of the framerate coming back to normal and it drops on certain events like volume control and control center. Sometimes the game starts on good framerate and sometimes starts on very low framerate.

All these framerate issues happen only when I build the game in portrait orientation, the same game runs fine in landscape orientation.

I am on "iPhone X" and iOS "11.2.2".
Other iOS devices don't have this issue.

Please someone tell me why this strange fps issue is happening.