The bug here is actually a display one not part of Kangaroo.
Whenever a point in Grasshopper gets far enough from the origin, the preview display completely stops working until you refresh the viewports (I usually just click the 4-viewports button).
With pressure, because it is proportional to area, it can cause objects to inflate indefinitely if it is strong enough to overcome the tensile resistance of the mesh. This is physically correct for a constant pressure.
If you want a pressure force which diminishes as the volume increases, so the mesh tries to reach a target volume, then use the ‘volume’ goal.