IPv6 DNS servers are not advertised to Cisco AnyConnect clients

I have this in ocserv.conf:

dns = 172.16.0.1
dns = 2001:470:8a59::1

yet see this in the log:

Jan 15 10:54:08 igor ocserv[15332]: worker[rhester-laptop]: 172.16.0.1 adding DNS 172.16.0.1

and the (Windows 10 client, using Cisco AnyConnect) indeed receives only the first DNS server.