I value integrity, especially when communicating results. This is shameful.
(Disclosure: I worked on JPEG XL)
“For progressive, I think it's better to show the user something that's obviously a preview, but still has enough detail to be able to know what the picture is of. If you're showing the user something that they may mistake for complete but poor quality, that's a bad experience.”
-jaffathecafe
That's what UI is for - show an indicator that it's not complete and avoid the confusion, bringing back to the original issue
But also, that's not "obviously a preview", so you have the same potential confusion here
As to bad experience, that seems like a legit personal preference, but disturbingly un-nuanced and absolutist if intended to apply to everyone.
When on a train with spotty wifi or frequent tunnels, I would actually prefer to see whatever arrived. Someone on prepaid data might want to truncate at ~100 KB, no matter what the website owner or browser PM thought.