Load Balancing on an MX Security Appliance is designed to round-robin connections between both WAN uplinks, thereby balancing traffic load between the two. NAT rules, meanwhile, are intended to map a certain public IP to one or more internal IPs, so traffic to/from the internal device(s) will always use that public IP.
When the two features are used in conjunction, load balancing will cause outbound traffic to go out of either interface, even if there is a NAT rule in place. This can cause issues sending traffic from an internal IP denoted in a NAT rule.
The following instructions explain how to use Uplink Preferences to ensure that 1:1 NAT or 1:Many NAT traffic uses the appropriate interface:
Click Save changes.
Note: Additional uplink preferences will need to be configured for each NAT rule. For a 1:Many NAT rule, each internal device will need to be included in an uplink preference. To simplify the list of uplink preferences, a subnet can be specified for Local IP range instead of a single device.