It's not a bug. In a recent patch I noticed that when Twintania was pulled, if I used any kind of self buffs on myself (eg. Raging Strikes) before the OT established aggro on the adds, they would all flock to me. It's pretty common for a bard to pop 10 million buffs before they begin attacking so either make sure the OT uses his buffs first or just have the bard wait a second or two for threat to be established, as I am sure that powerup buffs now establish some low amount of enmity.