I checked 16th May for EZE-SYD direct flights and saw no LA codeshare, yet QF18 does operate this day. This can been seen with
www.seatcounter.com . Nor is it showing on LAN.com for this date.
I have now checked 29th June and LA4518 is indeed showing.
Further checks show no LA4518 on 18th May. However there are the said codeshares on May the 20th, 23rd, 25th and 27th.
So perhaps LAN/Qantas are resuming their codeshare or maybe the codeshares for the 16th/18th are sold.