Fortunately I haven't detected any water ingress in the tailgate myself. However, it did occur to me could the water be entering through the numberplate fixings when open in the rain. The contour of the panel would mean water would soon pool and if inadequately sealed allow water to ingress over time. When the tailgate is closed the amount of water the fixings experience would be minimal, protected by the overhang and the numberplate itself. Similarly, the handle recess could be another weak point.
Rather negates the benefit of using the tailgate as shelter from the rain.