What's different about the subnet/interface that can reach it?
When you try to reach it from the working subnet check the states that are created.
Compare that with states created when trying from a failing subnet. Check the firewall logs.
Connection refused instantly implies something is responding that it's blocked. The default pfSense block rule doesn't do that. So it may be incorrectly routed or denied at the target device.
Your block 1918 destinations would block this connection since NAT happens before firewall rules. The NAT reflection rules should translate the destination from the CARP/IPAlias VIP to the internal server IP and that would be blocked.
Are you trying to connect using an FQDN? Does that resolve to the public VIP?
Steve