Ok, if you only have a firewall rule with the OpenVPN gateway set it will force all traffic out that way which will break connectivity to the LAN.
Add a rule on the new interface above any rules with a gateway set to pass ping traffic to the LAN.
Otherwise check the firewall logs. Check the state table while you're pinging.
In the forums, similar cases of pfSense ignoring the static routes have been discussed in the past. In many cases, people mentioned using a floating rule to override this behaviour which did not work for me.