So I updated to iOS latest Anyconnect. Which claims now support ipv6. But after some testing, it seems like the iOS app will not tunnel ipv6 traffic when the server doesn't support ipv6 in the first place. But instead of dropping all ipv6 traffic, all ipv6 traffic will go through local connection if your local connection support ipv6.
Not sure if it is a intentional design choice or serious bug? While I haven't looked very deep into it but I doubt ipv6 connections got dumped to local is encrypted in anyway. Mac version doesn't seem to be have this problem and properly dumped ipv6 link when the vpn server doesn't support ipv6.