When I right-click on the scatter plot and select "add to layout", it adds the scatterplot but with a black box surrounding it on three sides (with the scatter plot in the upper left corner of the box). Resizing does nothing because it just reshapes the whole box and keeps the same black background with it. I have tried re-creating the scatter plot multiple times, have restarted my computer/ArcGIS multiple times, have even started again with the same original data and it still happens.
Graphics in ArcMap have never been predicatable.
Maybe not ideal, but you could Export as a graphic (PNG/SVG are usually good) and insert it with Insert Picture, or Screenshot, Crop in PPT then Ctrl-V it into the layout.