It makes sense that boost would build until a certain point, and then steadily drop with RPM. Because, the blow off valve would be open when you aren't on throttle, and then, you run out of gas and floor it. The engine is still turning, and pumping air. It spools the turbo up once the blow off valve closes. Since you are flooring it, but no fuel is being added, it's pretty much air alone that is keeping the turbo spinning. It stays at maximum boost, wastegate is opened, but then the engine obviously is slowing down from internal friction, and the friction of the tires on the road, wind resistance, etc etc. So since the RPM is dropping, less air is flowing through the engine, and the boost drops. Until eventually the engine stops, and therefore there is no more boost.
EDIT: OH, I tested it out in LFS and I see what everyone is talking about. The boost actually increases from the point at which you cut the ignition, and when the RPM is dropping. That obviously is incorrect.