[Vuejs]-Object reactivity of complex object

The issue appears to be that the Smooth dnd library you are using is not updating the array of blocks that you are passing to it, it is likely making a copy of the array internally. So when you change the position of the blocks by dragging and dropping, you are not changing your blocks array, just the internal copy.

Looking at the Smooth dnd documentation, if you wanted to access the modified array you could try using the drag-end event handler:

onDragEnd (dragResult) {
  const { isSource, payload, willAcceptDrop } = dragResult
}