Try this :
Create a file “A” with nested blocks.
Open a new file “B”, and attach file “A” into it, thus creating a worksession.
Select the nested block and copy it somewhere else.
This creates a block and possibly new layers in file “B”.
But if you Blockedit the newly created block, you realize that the sub-blocks are still in file “A”.
This creates all sorts of bizarre issues like shown below and proves once again that the block logic in Rhino is flawed and un-reliable.