I'm experiencing the same problem. It is seeing the Mac App portion but not the iOS portion.

I just did an upload to try to see if the problem is still there. The upload WORKED! Thank you Apple Support for looking into it and fixing it so quickly.

I just made one screen shot with Pixelmator for the required size and added it for the iPhone 8 screen shot. From my understanding iPhone 8 is not supported with iOS 17 so that size screen shot shouldn't be needed for an iOS app that require iOS 17 as a minimum. I have successfully submitted an app to the App Store 3 times now. Also, I am not sure that the current submission process requires the iPhone 8 screen size. I left mine in and it seems to be happy with it. However, I did put a note in the App Review notes to mention the placeholder screen shot added for the iPhone 8.