I wanted to copy and paste a background shape from the top of a diagram to another space further down the page. When the background shape (magnetic) was pasted on top of another background shape (magnetic), it wanted to grab not just the stuff assigned to it, but everything behind it, including two other background shapes.
With the exception of the magnetic shape grabbing other shapes, that all sounds like it is working the way it is designed to. Magnetic shapes grab everything that overlaps their border area. There is no “contains”, just “overlaps”, if that makes sense. The original items never belonged to the first shape, and if the second shape overlaps them, it will carry them.