Hear hear.
It's worth noting that this isn't just related to the back button. If you go into your profile, select favorites, sort by newest, then quick questions, then profile / favorites again, it will have been reverted to votes again.
Just because it's related to it being an AJAX call doesn't mean it's not a bug, IMO. It's defying the user's expectations (partially set by sort order elsewhere being sticky). It may be harder to work around due to being an AJAX call, but it can't be impossible... the AJAX call could set a flag in the user profile, for instance - and that flag come down to the page when the user fetches the list of favorites next time.
I suppose it could count as "not a bug" in terms of working as designed, but in that case I'd say the feature is not as well designed as it might be :)
(I don't mean to sound whiny in all of this... it's not like I don't appreciate all the cool stuff the team does :)