Not sure. We'll have to check into it. CB is extremely picky with how things are done, and we have a lot of workarounds to get around that pickiness. The avatar is stored separately from the profile data during the registration process, so it's possible that CB is seeing it not all complete at the same time and rejecting the user.
Hopefully, marking it as not-required is acceptable for now. We will look into it further, but I'm not sure how long it may take to get you an answer. We have plans to completely overhaul our profile integration for the v5.0 release late this year or early next. That will make it easier to add workarounds for issues like this. If we can implement a fix sooner, we definitely will. This is an issue we simply haven't heard of yet, so will need to do a bit of testing first though.
Thanks,
Alex
CB and Facebook registration issue
