Interesting: on my 2020 iPad it displays fine in Safari
iPhone XS (iOS 15.1 / A12), displays a black box.
2020 iPad Air (iPadOS 15.1 / A14), displays image perfectly.
Interesting if it's hardware assisted.
If their "2020 iPad" was actually a 4th gen Air (A14 Bionic) or 2nd iPad Pro (A12Z Bionic), it could still be a hardware decoding difference.
If I had to guess, A14 devices aren't using the hardware decoder any more for some reason, and that's why it works.