Prototype preview radius stretched out of proportion

This only started happening recently.

Where the main frame has a border radius, in prototype preview, fixed width, with device frame visible, the border radius of the frame is stretched beyond where it shows on the frame on the canvas, varying depending on the height of the frame. 

Radius barely reaches beyond the ‘9’ in the time yet on the preview you can see it stretched as far as the orange icon.

Is there a way to fix this without having to lose the radius on the canvas frame (which does work for the previewer but is not something I want for the canvas for other reasons).