I just hit 12 as a monk and, to my surprise, when I got Chain Attack 2, my Chain Attack 1 was replaced.. As I recall, chain attacks were to be used, as the name implies, in a chain! And their additional effect which buffs your next chain attack, of course, supports that, so.. Not sure what changed that'd throw it off.
I haven't logged out/in or anything yet, maybe it'll re-appear, but seems odd for such an issue to crop up!
edit: I logged out and back in.. now I have Chain Attack I, but not II! Something seems bugged with the concurrence of these abilities' availability.