ipsec : L2TP and site-to-site together

Manualy adding this nat rule solved my problem. Is it a normal behavior ? I mean having to manage nat routing for L2TP/SITE-TO-SITE VPN ?

rule 1 { description "VPN L2TP/SITE2SITE NAT" destination { address 172.18.5.0/24 } outbound-interface eth0 source { address 172.18.10.0/24 } translation { address 172.18.4.1 } }