Interesting, as that makes it sound like this is a macOS issue or something. But according to the Wikipedia page, it is supported:
WebKit added AVIF support on 5 March 2021. Safari for iOS 16 and macOS Ventura added support for AVIF; iOS 16 was released on 12 September 2022[15] and macOS Ventura on 24 October 2022.[16] Safari 16.4 retroactively added AVIF support for macOS Monterey and macOS Big Sur.[17]
https://en.wikipedia.org/wiki/AVIF#Support
Though as I have seen with WebP, just because something claims to support an image filetype doesn't mean it supports animations in that filetype.
EDIT: It doesn't work for me either when I view it from your instance (web browser on Android or Linux). You have to click on the image to open it in a new tab, then it plays back animated there. I guess animated AVIF is just another thing that PyLova supports that PieFed doesn't.