Giffen Leaving Legion of Super-Heroes
However, now #18 appears to be Giffen's last as Bleeding Cool says that Giffen walked off the series.
No details are known, but BC mentions similar editorial differences with other creators have caused ruckus, which could be the cause here.
Gail Simone and Batgirl comes to mind in addition to DC Comics supposedly telling their other creators to keep a lid on things or else.
It's also said that this isn't affecting Giffen's work on his other books, with his new cosmic book, Threshold, having launched this month.