If you add elements to a group while you are in a workset that's differs from the workset of the group itself those new elements do not get put in to the workset that the overall group is in. Instead, they are in a seperate workset. Even though everything within a group should all be in the same workset you can have elements in a group that are on different worksets.
All elements within the group will be on the correct workset for any copies of the group, but the group that you edited is incorrect. If you are using workset filters than these elements will appear differently than the rest of the group.
The bug does not apply to elements that are added to a group with the add tool. It only applies to new elements that are drawn in a group.