I'm not having this issue with my 7 Plus but I am going to suggest doing a hard reset of your phone and see if that doesn't resolve the issue. Hold the power and volume down button together until you see the logo. Then release the buttons and wait for it to finish. Hopefully this will solve the problem for you. Also make sure the iOS is up to date on your phone. Thanks!
Camera app in itself has a ton of bugs. Having all kinds of focus issues here. Some pics come out perfect, others grainy/blurry. Super inconsistent image quality. Just never know what your going to get. Same for portrait mode. Sometimes it works awesome, other times I can't tell it applies any blurring. I realize some settings are "harder" than others... not just in reference to portrait mode but in general I REALLY expected more of my 7+ camera.