My favourites, when showing "Date Added", do not actually sort by the date I added them to my favourites. Most do, a handful do not.
Not entirely sure how to consistently reproduce as it just started happening and I can't seem to change its behavior, but it's happening right now for me.
My main first avatar, which I always keep up the top by unfavouriting and refavouriting, is now bumped low.
Previously, this avatar would always be up the top of my list.
The 2 highlighted are usually near the top.