I did a little research myself because I’m too affected by my avatar not changing.
The forum software they are using here caches user avatars. Once they are synced from myTeamSpeak - which is this forum’s single-sign-on provider - they are apparently kept until the SSO notifies the forum that it needs to refresh the avatar. It probably doesn’t do that right now.
The fun thing is: I don’t remember ever setting an avatar, especially since I’m not in the beta. So I guess Gravatar may also be involved and I read that the forum software supports that as well. And I also found evidence that it may be too affected by this very same caching issue.
Good news is: TeamSpeak can probably at least fix the issues of myTeamSpeak avatars not updating. For me who is not in the beta and probably affected by “Gravatar caching”, I guess the forum software needs to be patched so it allows users to refresh their cached Gravatar.
By the way: Gravatar supports content rating. You can upload different avatars for different audiences. If TeamSpeak was to adapt the same feature, it could show different avatars for different audiences.
