Why are classes moved when I move a class diagram?

I need to move a class diagramm into another package. The diagram contains classes from different packages. When I am now detach the diagram and attach it to a package, the classes are also moved.
Ho can I prevent this ?

Hello Martin,

Moving a diagram form one package to another, but without moving its containing models seems to be something very abnormal. In fact, our assumption is the diagram under a package should mainly modeling the things under that package. According to your description, it seems become a diagram under package A is modeling things mainly under package B. This looks very strange. Would you mind tell me more details about why you want to do in this way?

Best regards,