Problems when copying to a custom group

One of the things with custom groups that helps me is the ability to drag and drop into a custom group. However, occasionally when I drag a selection (usually a large number of citations) from another library into a group they will not be added to the group.

They are added to the all references, and if I go there they are all selected still, but they do not go into the group. This doesn’t happen all of the time, so it makes it even more frustrating. Is this a known bug?