pfSense

When you create a dual stack OpenVPN server with the following settings:

Gateway creation: both
IPv4 (legacy ip) tunnel network: 192.168.1.0/24
IPv6 tunnel network: 2001:db8:100:c07::/64

The interface is created like this:

 OVPN_UDP (opt5)  -> ovpns1  -> v4: 192.168.1.1/24
                                v6: 2001:db8:100:c07::1/64

ie for both stacks the address "1" is assigned to the ovpns interface on the firewall, and other addresses are used for clients.

There are then two gateways created under routing:

OVPN_UDP_VPNV4        OVPN_UDP    192.168.1.1    192.168.1.1    Interface OVPN_UDP_VPNV4 Gateway       
OVPN_UDP_VPNV6        OVPN_UDP    2001:db8:100:c07::2    2001:db8:100:c07::2    Interface OVPN_UDP_VPNV6 Gateway

As you can see the legacy gateway is .1, which is the firewall, so the gateway monitoring pings the firewall itself which responds and the gateway is marked as up.
The v6 gateway is :2 whereas the firewall is :1, and since there is no client using the :2 address attempts to ping this address fail, which results in the gateway being marked as down.

This is then visible under status->gateways where the legacy gateway shows as online and the v6 gateway shows as offilne with 100% packet loss:

OVPN_UDP_VPNV4    192.168.1.1    192.168.1.1    0.048ms    0.024ms    0.0%    Online    Interface OVPN_UDP_VPNV4 Gateway     
OVPN_UDP_VPNV6    2001:db8:100:c07::2    2001:db8:100:c07::2    0ms    0ms    100%    Offline, Packetloss: 100%    Interface OVPN_UDP_VPNV6 Gateway