Information
Many services (NTP, SNMP, RADIUS, etc.) can be configured with a source-address option which allows you to statically configure the source address to use for communication. In those circumstances the source address becomes the one that is specified with the source-address argument (provided the address is a valid address specified on the interface of a router), otherwise default-address-selection influences the default source address selection.
Solution
Configure source address for syslog traffic.
user@host# edit system syslog
user@host# set source-address <IP_ADDRESS>