Am I correct in assuming that pretty much the only time your Facebook profile pic syncs as your avatar is during registration, and that's it? I notice I can't retrieve it with an already created account.
I did run the User Cache rebuild right after importing everyone from a vB 3.8 board. My own account, however, didn't seem to include any kind of avatar sync when I associated it with Facebook. However, I tried a test account with my daughter's Facebook profile, and it synced upon registration. But even here on Xenforo.com, I don't really see any way to have my avatar updated to my current profile pic. It just uses the one I had when I registered 2 years ago. There's Use a custom avatar or Use Gravatar. Nothing really about Facebook, other than my initial profile pic sitting in the first box as if it had been uploaded manually.
The avatar is being copied while the registration and that's it