I've just tested the same videos on an iPhone 13 with iOS 16.6.1 and loadFileRepresentation is working, so this looks like an iOS 17 specific issue.

This just occurred for me (Jan 2024) and all the images were showing as there. So I closed that browser window in Safari, opened a 'New Private Window', logged in, and checked all the screenshots. One of them was showing as grey (still processing) so I deleted and replaced it, I was then able to continue and submit the app update. So it's clearly a caching issue in the web app, opening a new clean (private) browser window showed the correct/real state of the screenshots.