It’s a problem in Chrome too.
Couple things to note: the blue and red rectangles are still Inkscape rectange shapes and haven’t been turned into paths. That can sometimes be an issue. IN this case though it isn’t. The margin around the bitmap is a bit tight in relation to the red rectangle, so there may be some interference, but resized the rectangles and that didn’t make a difference.
Have you tried loading the graphic separately and then uploading the rectangles for cutting/scoring, or start with the SVG of the rectangles and them import/upload/place the graphic in the operations space of the GFUI?
This is interesting.