AVIF is based on a video codec and stores a photograph in a very small file at good visible quality. It can also hold more tonal steps and wider colour than JPEG. Shop systems increasingly use it for delivery: they take the uploaded JPEG or PNG and serve the page as AVIF, which the seller does not need to produce or manage.
As an upload, AVIF is accepted by some channels and refused by others. It also turns up by accident: an image saved from a web page, such as a supplier's product photo, often arrives as an AVIF file because that is what the site delivered. Such a copy has already been reduced and compressed for display, and the original JPEG or PNG is the file to ask the supplier for.
When an AVIF has to become a JPEG for upload, the conversion keeps the loss already in the file and cannot bring detail back. Colour needs a look as well: an AVIF with wide or high-dynamic-range colour can come out flat or oversaturated in a converter that ignores this, so the converted file is compared with the original and set to sRGB in the same step.