I can confirm your report of a different behavior from earlier versions. However, I am uncertain of the full changes. For example, if I create a new group in Addressbook on a Mac with Mavericks, and then use that group in Mail on my older MP with Lion, then attempting to send to that group without expanding to show all its addresses will result in failure and a warning message that no recipient is specified. Yet groups created earlier in Lion will still send as before in Lion.
Furthermore, when I created a new Group in the contacts in Lion, that Group was never picked up in Mavericks on my other Macs. But the new group created in Mavericks is seen in Lion, but just requires expansion to show all name to then send.
More investigation required, but I am suspicious that more is involved.
Ernie