It’s actually not openvpn that causes the problem. If you call openvpn directly (from commandline or in a script) it works dual stack, the problem is in the connman plugin for openvpn (see Testing CLAT for IPv6-only mobile networks - #138 by jlaakkonen)
As a workaround i’m using a script to do the VPN setup, the script also shared in that topic but be aware it’s very very hacky.