It was meant to be the reunion of Team Levitz/Giffen. Two creators, both committed to Legion stories, both working on the book separately and together in the past, both with The Great Darkness Saga on their resumes. And reunited again for new Legion Of Superheroes stories, starting this month. However, I understand that, despite the solicitations of issue 19 in April listing Giffen as artist, he has walked off the book with the previous issue.
I don't know the details, but similar decisions from others recently have been over creative and editorial differences. I don't believe Giffen is leaving any other books he's working on for DC, that it's solely down to the circumstances of this project.
But odds are it will be discussed at the upcoming DC Creative Conference in Charlotte.