Hi mate, posted this in response to a bug report that I created for this, as well as one of the bigger reports on this. May be a temporary fix (not a guarantee it will work, it did for me):
I finally got this to work - unsure how though. I played Ryloth again, won as the defender, post round screen said challenge done. I stayed on for a couple of games as I was close to 100 kills as a hero ship, got that finished within a few games, and post round it said multiplayer milestone was at 24/26. Exited to main menu and The Defender was there as completed and to be claimed! I wonder if the other challenge completion ensured that one stayed as completed.
Still a glitch that it cant be done on a proper basis, but that may be a temporary fix. Try completing a different starfighter challenge in the same lobby too,
