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Are both running the same version of iOS?

Since 2019, iPads no longer run iOS, they run iPadOS. Of course, there's still a lot of overlap between the two but differences can creep in. A difference that shouldn't be relevant to JPEG rendering is iPadOS Safari loads the desktop versions of websites by default.

Both run 15.1. But apparently iOS vs iPadOS have differences.

What processor does your iPhone have? Maybe it's a hardware decoding difference. It renders on my iPhone with a A14 Bionic processor.

I just updated my original post. A12 Bionic is my iPhone (XS).

Interesting if it's hardware assisted.

If the comment you replied to meant iPad 10.2" 8th Gen by "2020 iPad," that also has an A12 Bionic.

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.

iPhone SoCs have had hardware JPEG decoding since basically forever. However, I'd be surprised if that hardware block suddenly got updated to support AC.

If I had to guess, A14 devices aren't using the hardware decoder any more for some reason, and that's why it works.

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