I've tried to set other photos as avatars with the same result. For whatever reason, once I hit Save, the picture disappears. I don't remember the exact date, but I'm sure I uninstalled and reinstalled the app within the week. I checked the Version number and it's the latest, Version 6.4.5.
I hope this is helpful. I really like the app and would like to get it to work as it has in the past. Many thanks!