So, if a group is off and I turn one of the "member bulbs" on when previously off and when their last "on" state was a different color/CT (so I might expect some group event for color), the only events I see for the group are:
This sounds consistent with what you said. If I send a command directly to the group (you can see the "raw" API commands as a Groovy Map in the bottom log entry below), I still don't get color information back. This is when the group was off but again previously on to a different color or CT, so I'd expect some event:
The top line is the eventstream data, stripped down to just the information for that particular group ID. (I did also check the entirety of the data and see something for every light in the group, with all the data I'd expect--just nothing besides on/off for the group itself.) The same is true whether I do this from Hubitat or the Hue app, though it's a bit more difficult to deal with the Hue app given its tendency to want to use XY data for color and CT.
It seems like this second thing is different from what you're seeing.
Not sure what to make of this, other than that hopefully we'll know more if/when they ever document this. 
A far more annoying problem for me is the tendency of the eventsocket to falsely report disconnects when, in fact, data is still coming in, so I'm never sure whether I can trust the current status...not sure if documentation will help with that. Ha.
