Mike said:
Hopefully later version to come they will make the rounded corners an option.
The rounded corners are an option. It's just difficult to set the radius
of curvature precisely. You could try creating a square of side the
radius that you want, aligning it exactly so that its edges match with
those of the rounded rectangle in the corner with the grab handle,
zooming in to maximum zoom, and then dragging the grab handle to the
appropriate corner of the square.