There seems to be a bug in the SNMP module in that it will only respond if the SNMP request came in on the IP address being used for the default route. Take the following configuration for example:
Interface Ether1:
IP address 66.133.12.4/28
VLAN Interface 102 on Ether1:
IP address 10.0.0.4/24
Interface Ether2:
IP Address 12.44.22.38/26
SNMP Enabled
No firewall rules. All firewall chains policies set to Accept.
Routing table:
only 0.0.0.0/0 to 66.133.12.1
Now you should be able to send an SNMP request to 66.133.12.4 or 10.0.0.4, but it will only work via 66.133.12.4.
This behavior exists in 2.8.26 and 2.9rc7.