I experienced the same issue after uploading numerous app preview videos and screenshots for an app submission. The work around I found was to go to the "View all sizes in media manager" page. Delete the last (10th) entry for all of my galleries. Then exit and navigate back to the same page. One of the device image galleries then showed a gray box at the end where the 10th image would be. I removed it, added back in the 10th images for my devices then submitted, and it worked. My guess is that Apple has a hard coded limit in the UI to not show more than 10 images. If an issue happens on upload, you'll never see the gray box to fix it if you already have 10 images. Note, the same issue may exist with App Previews if you have the max of 3 of them. I only had 2 app previews per-device so didn't experience it.