Selecting all actors and copying receives "Actor [name] is referenced by another Actor, do you really want to delete it?

Selecting all actors and copying receives "Actor [name] is referenced by another Actor, do you really want to delete it?". This seems to only happen if one of the actors is referencing another actor and the project also has sub-levels.

User Description:

Hi. I have for now 1 243 actors on multilayer scene. I select one, then I pressed ctrl+A to select all. Next I pressed ctrl+C to copy and then I've got "Actor actorname is referenced by another Actor, do you really want to delete it?"

I can paste them into some text editor after that, but why this question appear? I'm very sure that I pressed ctrl+C NOT X.

1. Open a new blank project
2. Windows > Levels
3. In the Levels window, Levels > Create New > Save
4. Back in the editor, press Ctrl-A to select all actors
5. Press Ctrl-C to copy (or Right-click > Edit > Copy)

Results: The "Actor LightSource2 is referenced by another Actor, do you really want to delete it?" message pops up on copy.

Expected: For the message to not popup on copy but only when this reference is actually being deleted.