For serious, general purpose programs when thumbnails are shown people expect them to correspond to the full images, and not something else. Windows explorer, android gallery, gnome file picker etc can't show made-up thumbnails, or people would send or delete the wrong images, or would have no idea what images do they actually have.

png/webp (just as jxl) can store thumbnail in their meta, but no operating system, gallery, browser etc uses that because it has no usage that is safe.

I am not sure how easy it is to create a jxl image that starts differently from how it ends. If it is super easy then I expect programs not relying on it for thumbnailing (especially that they already have a way to generate and store thumbnails). Maybe browsers will support progressive loading, but it has no use for local images.