One of the things I’ve noticed when using Copy/Paste in the GFUI is that if there are a lot of parts in the original, (like dozens of little dots) when you go to drag the original off, it’s easy to accidentally pick up some bits of other items as well.
This is avoided if the little bits are completely enclosed in a closed shape, but not if there is an open cut path partially surrounding them or if the shapes overlap. With that you just have to be extremely careful.
That triangular shaped piece has circles that cross over the orange cut line - the parts are not completely enclosed by the outer shape, so the shape is treated as individual paths and not a group. You could try deleting the little circles that cross over the lines and see if it doesn’t work a little better when copying and dragging.