I have had the same network-wide profile avatar for many years; it's a custom image which I installed nework-wide soon after I registered for Stack Overflow back in 2011. I still see that image e.g. on my Stack Overflow profile and my network profile but not, for instance, on this site, or e.g. on Super User
(The example originally linked to my Unix & Linux profile instead of Super User but I experimentally changed my profile back to the gravatar one there.)
I routinely update my profile presentation text every few months, and did so again a few days ago. Comments below seem to indicate that others who recently edited their profile are experiencing the same problem.
My preferred avatar is a Gravatar which is connected to my email address, which has always been the same (MD5 091f411d57db5be8298e057a32e5ad72). The replacement seems to be a generic identicon with a different MD5. I don't recall seeing this identicon as an option in my profile before:
You'll notice that the gravatar is still visible as an option, but I can find no way to replace it on all sites in one go. I switched back to the gravatar one on Unix & Linux and noticed that the help next to the "Save" button says
Your profile will be updated on all public communities.
but alas, that did not in fact happen. Do I really have to go over 170+ member sites and manually select my gravatar on each?