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This differential has been attributed to some of the drag reduction which is not present when a bulkhead is placed between the gate and leading edge of the tonneau.
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Who says it is not present? Even assuming the drag goes up (how much?) with a bulkhead, that might be the effect of other things, such as the shape the vortex is forced into assuming. With another truck, with different height and width of the box and shape of the wing, a bulkhead might make the drag go down. From my reading so far I get the impression this bulkhead effect is not a settled thing.
Let's imagine a sealed box was bolted to the front surface of the tailgate. If there was a pressure differential without it, then there must be one with it. It doesn't matter what pressure is inside the box, since it is sealed.
If it is not sealed, then the low pressure in front of the box is simply translated through the box to the front of the tailgate.
Indeed, the tailgate is itself a box. It has a front and rear surface.
Personally I don't buy this pressure differential doing anything, since it has the reverse effect at the front of the pickup box. Instead I think what happens is that the vortex simply transfers (more or less efficiently, depending on configuration) the air over the opening to the half-tonneau. Your adding the blister just helped this along by providing a surface for the airflow to easily attach to.
I'm getting the impression nobody has tried to give the vortex a cylindrical space to rotate in.