What does the Force Cloning option in the Create Bleed window do?
Create Bleed extends the original objects.
However in case of an image with a mask with some image pixels outside the mask, the mask is extended. In some cases (say for example an object on a white background and a mask on the outer contours of the object) pop-up white pixels are created.
Force cloning will force the copying of the CT pixels from the inside of the image to create the bleed, though there is original data available (white pixels).