when a group member logs out or change character, the group frames still shows me this character is "online". as far i remember, i didn't teleport and wasn't in loading screen at all.
that happens often (3-4 times each week) for no reasons. i mostly play with max. 4 players (= group frames), but maybe this issue is in raid frames too?
It seems that EVENT_GROUP_MEMBER_CONNECTED_STATUS does not always fire, so the callbacks that update the group/raid frames don't update properly. I haven't noticed a real pattern to why it happens, yet.
Honestly the grouping API in general is fairly buggy. Health desyncs, 1/1 health for players, the Unit functions can't always get the class data, etc.