Why do layers get mixed up during editing?

When I split a file in two and move the resulting piece on the left to the right, this piece gets on top of the right hand side layer and not below it as it should. The part of a file that was created earlier must not cover a part that was created later – except for the case when we select “Move to Front”. This had been working properly for 15 years. Why doesn’t it work in Nuendo 6? Editing now is impossible because of this bug.