I hope it's been solved after 2 years.
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So as it turns out, App Preview Videos are only for Mojave and later. It would have been nice if this was mentioned somewhere prominent inside of app store connect's video uploading interface, since according to StatCounter, 35% of Mac Users are currently *not* using versions Mojave and later, and therefore would not be seeing the videos.
Apple doesn’t monitor these forums so you may be better off emailing their support directly and linking to this post