Turn 2 & turn 5 will be reverted back to how it was in 2.15 per YoshiP.

Basically boss battles are done by a system called Gambit, like the FFXII Gambit system but they use it on the mobs. While they were making changes for 2.2 Bahamut content, it some how changed the mechanic where it detects the number of PC players in turn 2 and the process in which the monster targets was changed on turn 5 so they are working right now to get it reverted back to how it was in 2.15.

*Quick summary of what YoshiP posted here - http://forum.square-enix.com/ffxiv/t...=1#post1887585