I think at some point some avatars were converted to animated PNG’s through a process I’ve never been able to replicate, and I’ve never managed to successfully upload an animated PNG anywhere. You also might have older versions of some avatars still cached?
Every single part of image resizing and conversion has been broken and dysfunctional in a revolving sequence of fail for months. Whoever is responsible for this part of the software is pretty terrible at their job imo.