This will likely come as a surprise, but I’m actually thinking this may be a feature request. This appears to be the default expected behavior. When you moved the group into the other group and let it encompass nodes, those nodes will move with group they are within.
I am not sure how you can word the feature request such that you get the behavior you want without upending the existing default behavior. But I do understand where you are coming from.
…but I agree with @I-d-as: I’m not sure that this is a “bug” or that it can even be fixed. If you changed how this worked, you’d change how groups worked in general.