This one is IMHO a really nice, as it does not hurt but is a bug.
A contact “user” is in group “friends” and shown correctly as online
Right click onto “user” and click “move to” to actually move the contact to group “family”.
Now, when the group “family” is not shown due to nobody from “family” is online at the moment, “user” is gone as group family is not shown!
When activating “show empty groups” we get to see “user” correctly in “family” and even when delecting “empty groups” the group family stays visible as it is not empty now.
Conclusion: When moving a user to a new group, Spark should recheck whether a formally empty group should be made visible.