Well it depends on the meaning.
You do not provide any reason why the second monitor ip should not be unreachable.
Can you show system log and the screenshots of when this happens?
Also can you verify that the gateways are not in the same mac address!?
I have attached the screenshots of routes, arp, gateway and firewall rules of vlan3.
I have two gateways (adsl modems): 192.168.1.1 and 192.168.1.5, the wan interface is 192.168.1.3 with selected gw 192.168.1.1.
In routing table i can see that:
188.8.131.52 192.168.1.5 UGHS 0 162674 vr0
but if i do a traceroute from the firewall to 184.108.40.206 (the monitor IP of the second gateway) i have:
# traceroute 220.127.116.11
traceroute to 18.104.22.168 ( 22.214.171.124 ), 64 hops max, 40 byte packets
1 192.168.1.254 ( 192.168.1.254 ) 1.583 ms 1.320 ms 4.315 ms
2 static-213-205-... etc.
As you can see from the screenshot, the routing table is not considered and all packets pass always through the default route.
In the firewall rules of vlan3 (OPT3) i have selected the backup gateway 192.168.1.5 but if i do a traceroute from a client in this vlan, i have same response of above.
In other words, all the packet pass through the gateway defined in wan interface.
I hope that my explanations are clear.