Tested with firmware 8.1 beta 4, but I am pretty sure this has been true for a long time.
If, I forward port 1111 (for example) to port 2222 (for example) and then create an inbound firewall rule that logs all incoming requests to the target LAN side server, the firewall log only reports one of the two ports.
I do this all the time because I use a remote control program that needs an open port and the inbound firewall rule is an audit log for me.
Below is a sample log entry. That the WAN side request came in on port 1111 is not mentioned.
[ 5301.837118] Firewall: Allowed CONN=WAN1 MAC=00:1a:dd:f9:8a:a1:f4:92:34:10:d7:9e:07:00:41:00:00:24
SRC=[wan IP] DST=192.168.50.50
LEN=52 TOS=0x00 PREC=0x00 TTL=127 ID=16544 DF
PROTO=TCP SPT=21463 DPT=2222
WINDOW=64240 RES=0x00 SYN URGP=0 MARK=0xa