This makes sense and I thought, "Ah ha!" Alas, after pasting and running the search, I get a 0 results search reply. When I run a statistics report, it says I have the same number of thoughts as before the paste. I even tried deleting the brain (it was new, just for this) and starting from scratch. Same thing.
zenrain wrote: I just tested this and received the same message. The child doesn't appear to be linked, so it doesn't appear they were input. After pasting, do a search for one of the thoughts (I just typed in "sat"), then activate it, then drill up. Once you get to the Year Planner 2018 thought it will appear to be correctly linked to the active thought when you pasted. I'm guessing restarting would correct it also (but don't do this if you want to undo the action).
So at least for me it was a display bug that needs to be resolved.
If you have done this multiple times, you might try undo. Or Just locate the parent, use Edit > Select Related Thoughts to get all of the child thoughts and delete them, then re-do.
Tested on macOS.