Black Shadow on top right of screen

Has anyone noticed a brief circular black shadow appear at the top right of the screen when exiting an app on iPhone 6?

 

Looks a bit like this! It doesn't happen all the time but very noticeable when it does happen, which is quite frequently. I've submitted a feedback report to Apple so!!

 

No I've just done a quick photoshop to illustrate it. It only appears for a second or so as the screen transitions to the home screen. So it's like a flash.