I’m not sure if you saw the second “Note” on the heat sealing page, but what you’re describing could be normal. All of the seams in the packrafts are designed to avoid that kind of peeling force – they’re either shear forces only, as in the areas where the tubes attach to the perimeter of the floor, or the area that would feel a peeling force is backed with a reinforcing strip on the inside of the weld so it would have to peel in two opposite directions at the same time (as where the tubes attach to the floor at the cockpit’s perimeter.
It’s possible you received a defective piece of fabric, so if in doubt, test the strength of those types of welds, but it is normal for the TPU-fabric bond to be weaker in the floor fabric compared to the tube fabric.