While I was using the Liquid Rescale GIMP plugin I tried to remove an area of width from the image I but noticed nothing was removed from the area selected in the discard mask. I turned on the seam layer creation and confirmed that nothing was being taken out of the discard mask. Not only was nothing being removed from the discard mask, pieces of the preserve mask were removed Everything I was using can be found in a Google Drive folder that I can email you a link to as I am not allowed to post it here.
GIMP 2.8.16
Windows 7