ICMP Error packets are not translated according to NAT rulebase
When it comes to the handling of ICMP error packets, in version R65 and below, the NAT rule base was inherited from the internal packet.
This means that the same NAT decisions would have been taken for the internal packet and the ICMP IP header, and there was no way to configure explicit rules for the ICMP Error IP header itself.
Starting in version R70, by default, it is possible to configure explicit NAT rules for ICMP Error packets. However, for routing decisions, an ICMP Error packet will go over the NAT rule base in the Outbound Chain and only after the Destination address has been translated according to the internal packet NAT rule base.