Try this. Select the "Advanced" tab on the photo export dialog to export photo, scroll down the photo export dialog to "Image Sizing" section, and then expand the section.
Check if there are any grayed-out pixel values in the Width and Height cells for the "Resize to fit" option.
If no pixel values exist in the aforementioned cells, select the option "Resize to fit", but do not keep the original resolution, and fill in any random pixel value for Width and Height.
After you've filled in the values, select the sizing option "Keep original photo resolution" again and then export the photo.
If that doesn't work, suggest that you contact technical support:
https://membership.cyberlink.com/support/service/technical-support.do
David