I created a Group 2.0 which was 5 Zigbee outlets (all Ikea Tradfri). I ticked the "enable Zigbee group messaging" box. All worked great - superfast with zero popcorn effect.
I then added another Tradfri to the group but the new one didn't switch on or off with the group.
I found that if I went back into the group settings, unticked the "enable Zigbee group messaging" box, hit done, and then went back in and then ticked it again, followed by done, all worked OK.
So it looks like maybe the Zigbee group part of the rule isn't getting updated correctly if you add a unit?
Running 2.0.6.112
