This issue occurs with any items being dragged and dropped onto a canvas. It will continue creating items even though the canvas is locked.
For like a couple of weeks I’ve thought that canvas was broken and that maybe it was related to the Graph drawing bugs, but nope, the canvas was just locked.
I think a good usability improvement is that if you drag and drop something onto a canvas that it displays a transparent lock icon overlay or pulses the lock icon itself.
This of course would also imply a solution to the addition of items to a locked dashboard!