I'm using fastlane's deliver to upload screenshots generated by fastlane's snapshot. The HTML preview looks correct and the upload script runs without errors for all the localized versions. Yet, AppStore Connect just shows the english screenshots for the localized versions. Without any error messages and the default configuration, I am a bit lost as to where the issue might be.
Running the exact same script again, using the exact same screenshots, it now works. I don't know why, but maybe someone else finds this useful.