I submitted an app update with new screenshots to showcase the added functionality. I took these screenshots on both an iPhone and an iPad, then used a service to generate the required variants for all device sizes needed by the App Store.
My app was rejected because the generated screenshots for the iPhone's 5.5" models (iPhone 6S+, iPhone 7+, iPhone 8+... all dating pre 2017) did not match the app's functionality. I was confused and assumed it was a mistake on their part.
However, I eventually realized the issue: the screenshot generator had cropped the larger screenshots to fit the smaller device screens, which was fine, but the tab bar at the bottom didn't display correctly when the device bezel was added.<p>TBH I was quite annoyed I had go and fix them manually but I find the attention to detail (of some of them) mind-blowing.