$currentgroup = groups_get_activity_group($cm, true);
Does not appear to be saving the current group in the session var any more. I find that if I change the group to any group and then load the page again without the group=x part of the url then the current group is reset to int(0) or 'All Participants'
I see this behaviour both in the forum module and the quiz overview and other reports.
I'm assuming this is related to Petr's recent SESSION handling changes and so assigning to him.