Create Start->Task->End inside of swimlane
Add to the process sub-process (for example embedded, event)
Select Start->Task->End nodes by Shift+click and move to the sub-process
Actual Result
Nodes will be hidden under sub-process, but sequence flows will be still visible. When you release mouse button, all nodes will be shown as expected.
Expected Result
All nodes will be visible during drag and drop as well.
When I drag and drop nodes from one container to another for some cases nodes are hidden under the container.