It's not really a bug but i'll try to explain better.
Create 2 groups. In one of the group add an event with a sub-event and fold it.
Then move the group containing the event in the other blank group.
You will see that the group you moved automatically unfolds itself as well as the event.
I would like non unfolding events when doing this manipulation. Because when you have 100 of them with different subevents level it quickly becomes messy..