Did you happen to resolve this? I'm running in the exact same issue, it seems that the Assets Pack is not reachable in the review environment, but I'm not having any issues in TestFlight. Apple Support didn't answer to my request either.

I also had the same problem, I tested the app with TestFlight and it works fine, even the fallback download of the assets after the installation. But the reviewer seems to fail to download the assets. It looks to me like the reviewer environment can't access asset packs. Also, mine is a MacOS app, how can I disable background downloads on my mac so I can test that behavior more realistically?