Export to JPEG or PNG is not working correctly on Surface Pro 4
The update to Plumbago has been released. This should address the issue that you were seeing.
Thanyavuth Akarasomcheep commented
I think background in each page is fix size.
Surplus will be black when I save to PNG.
Plumbago Support commented
We are aware of this issue and looking to have it fixed in next release. Hope to release very soon.
Run in the same issue. On Export the background of the image is partly white and partly black. It looks like it's the overall image has a resolution of 3830x2550 but only a area of 3000x2000 is white.