DELMIA Process Planning - drag & drop single operation from workplan to system

Dear DELMIA Community,

I observe a behavior while drag&drop of operations from workplan to process system, which I can't explain.
A single operation in sub structure of my workplan shall be assiged to a process plan (green). Unfortunatelly the parent operation is taken over evwerytime too (red). What is the reason for? (or is it a bug?)
Thank you 
Thomas Degen (TECHNIA)